Inertia valve fluid damper with isolated inertia mass

ABSTRACT

A damper having a valve, including a moveable inertia mass, movable between an open position and a closed position to selectively alter the compression damping rate of the shock absorber is described. The valve and especially the inertia mass are substantially isolated from the effects of the rebound and compression fluid flows.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/378,091, filed Feb. 28, 2003, pending, which is acontinuation-in-part of U.S. patent application Ser. No. 10/043,079,filed Jan. 9, 2002, issued as U.S. Pat. No. 6,581,948, and U.S. patentapplication Ser. No. 10/042,767, filed Jan. 9, 2002, issued as U.S. Pat.No. 6,604,751, which claim priority from U.S. Provisional PatentApplication No. 60/329,042, filed Oct. 12, 2001 and U.S. ProvisionalPatent Application No. 60/316,442, filed Aug. 30, 2001, and thisapplication also claims priority from U.S. Provisional PatentApplication No. 60/451,303, filed Feb. 28, 2003 and U.S. ProvisionalPatent Application No. 60/451,318, filed Feb. 28, 2003, the entiretiesof which are hereby expressly incorporated by reference herein and madea part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to vehicle suspensions systems. Moreparticularly, the present invention relates to acceleration sensitivedamping arrangements suitable for use in vehicle dampers (e.g., shockabsorbers, struts, front forks).

2. Description of the Related Art

Inertia valves are utilized in vehicle shock absorbers in an attempt tosense instantaneous accelerations originating from a particular portionof the vehicle, or acting in a particular direction, and to alter therate of damping accordingly. For example, the inertia valve may beconfigured to sense vertical accelerations originating at the sprungmass (e.g., the body of the vehicle) or at the unsprung mass (e.g., awheel and associated linkage of the vehicle). Alternatively, the inertiavalve may be configured to sense lateral accelerations of the vehicle.

Despite the apparent potential, and a long history of numerous attemptsto utilize inertia valves in vehicle suspension, commercial inertiavalve shock absorbers have enjoyed only limited success. Most attemptedinertia valve shock absorbers have suffered from unresponsive orinconsistent operation due to undesired extraneous forces acting on theinertia valve. These extraneous forces may result from manufacturinglimitations and/or external sources and often inhibit, or even prevent,operation of the inertia valve.

Further, there are currently no commercially available inertia valveshock absorbers for off-road bicycle, or mountain bike, applications.The problems associated with the use of inertia valves, mentioned abovein relation to other vehicles, are magnified in the environment oflightweight vehicles and the relatively small size of mountain bikeshock absorbers. Therefore, a need exists for an inertia valve shockabsorber that can be commercially produced, and provides responsive,consistent performance without the problems associated with priorinertia valve designs.

SUMMARY OF THE INVENTION

A preferred embodiment is a shock absorber comprising a first fluidchamber, a second fluid chamber and a fluid circuit connecting the firstfluid chamber and the second fluid chamber. An inertia valve includes aninertia mass movable between a first position and a second position. Theinertia valve permits a first rate of fluid flow through the fluidcircuit in the first position permits a second rate of fluid flowthrough the fluid circuit in the second position of the inertia mass.The second rate of fluid flow is non-equal to the first rate. A leadingsurface of the inertia mass when moving in a direction from the firstposition to the second position defines a leading surface area. A ratioof a mass of the inertia mass to the leading surface area is greaterthan about 130 grams per square inch.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber and a fluid circuit connecting the firstfluid chamber and the second fluid chamber. An inertia valve includes aninertia mass movable between a first position and a second position. Theinertia valve permits a first rate of fluid flow through the fluidcircuit in the first position and permits a second rate of fluid flow inthe second position. The second rate of fluid flow is non-equal to thefirst rate. A ratio of a mass of the inertia mass to a volume of theinertia mass is greater than about 148 grams per cubic inch.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber, and a fluid circuit connecting thefirst fluid chamber and the second fluid chamber. An inertia valveincludes an inertia mass movable between a first position and a secondposition. The inertia valve permits a first rate of fluid flow throughthe fluid circuit in the first position of the inertia mass and a secondrate of fluid flow in the second position of the inertia mass. Thesecond rate of fluid flow is non-equal to the first rate. At least aportion of the inertia mass comprises tungsten.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber, and a fluid circuit connecting thefirst fluid chamber and the second fluid chamber. An inertia valveincludes an inertia mass movable between a first position and a secondposition. The inertia valve permits a first rate of fluid flow throughthe fluid circuit in the first position of the inertia mass and a secondrate of fluid flow through the fluid circuit in the second position. Thesecond rate of fluid flow is non-equal to the first rate. The inertiamass comprises a first portion and a second portion. The first portionis constructed from a first material having a first density and thesecond portion being constructed from a second material having a seconddensity, the second density being greater than the first density.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber, and a fluid circuit connecting thefirst fluid chamber and the second fluid chamber. An inertia valveincludes an inertia mass moveable between a first position and a secondposition. The inertia valve permits a first rate of fluid flow throughthe fluid circuit in the first position of the inertia mass and a secondrate of fluid flow in the second position. The second rate of fluid flowis non-equal to the first rate. The inertia mass includes a collapsiblesection defining at least a portion of an external surface of theinertia mass. The collapsible section has a first orientation when theinertia mass is moving in a first direction from the first position tothe second position and a second orientation when the inertia mass ismoving in a second direction from the second position to the firstposition. The inertia mass has a first flow resistance when thecollapsible section is in the first orientation and a second flowresistance when the collapsible section is in the second orientation.The second flow resistance is greater than the first flow resistance.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber, and a fluid circuit connecting thefirst fluid chamber and the second fluid chamber. An inertia valveincludes an inertia mass moveable between a first position and a secondposition. The inertia valve permits a first rate of fluid flow throughthe fluid circuit in the first position of the inertia mass and a secondrate of fluid flow in the second position. The second rate of fluid flowis non-equal to the first rate. The inertia mass includes first andsecond opposing end surfaces oriented generally normal to a direction ofmotion of the inertia mass and a side wall extending between the firstand second end surfaces. The inertia mass additionally includes at leastone movable, annular skirt extending from the side wall. At least anouter portion of the at least one skirt moves toward the side wall whenthe inertia mass moves in a first direction and moves away from the sidewall when the inertia mass moves in a second direction opposite thefirst direction. The at least one skirt increases a fluid flow dragcoefficient of the inertia mass when moving in the second directioncompared to the drag coefficient of movement of the inertia mass in thefirst direction.

A preferred embodiment is a method of delaying an inertia valve within ashock absorber from returning to a closed position after an accelerationforce acting on the inertia valve diminishes. The method includesproviding an inertia mass movable in a first direction from a closedposition toward an open position of the inertia valve in response to anacceleration force above a predetermined threshold and movable in asecond direction from the open position toward the closed position ofthe inertia valve when the acceleration force is below the threshold.The method further includes configuring the inertia mass to have a firstfluid flow drag coefficient when moving in the first direction. Themethod also includes providing the inertia mass with a drag memberconfigured to increase the fluid flow drag coefficient when the inertiamass moves in the second direction to delay the inertia valve fromreturning to the closed position until a period of time after theacceleration force is reduced to, and remains, below the threshold.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber, and a fluid circuit connecting thefirst fluid chamber and the second fluid chamber. An inertia valveincludes an inertia mass and a stop. The inertia mass is movable betweena first position and a second position. The inertia valve permits afirst rate of fluid flow through the fluid circuit in the first positionof the inertia mass and a second rate of fluid flow through the fluidcircuit in the second position of the inertia mass. The second rate offluid flow is non-equal to the first rate. One of the inertia mass andthe stop defines a pocket for receiving the other of the inertia massand the stop in the second position of the inertia mass. A first refillpassage connects the second fluid chamber and the pocket and restrictsfluid flow therethrough from the second fluid chamber to the pocket toprovide a delay in movement of the inertia mass toward the firstposition. A second refill passage connects the second fluid chamber andthe pocket and a pressure actuated valve substantially prevents fluidflow between the second fluid chamber and the pocket through the secondrefill passage below a predetermined threshold pressure differentialbetween the second fluid chamber and the first fluid chamber. Thepressure actuated valve permits fluid flow between the second fluidchamber and the pocket through the second refill passage at, or above, apredetermined threshold pressure differential between the second fluidchamber and the first fluid chamber, thereby reducing or eliminating thedelay.

A preferred embodiment is a method of delaying an inertia valve within ashock absorber from returning to a closed position after an accelerationforce acting on the inertia valve diminishes. The method includesproviding an inertia mass movable in a first direction from a closedposition toward an open position of the inertia valve in response to anacceleration force above a predetermined threshold and movable in asecond direction from the open position toward the closed position ofthe inertia valve when the acceleration force is below the threshold.The method further includes providing a first delay force tending toresist movement of the inertia mass in the second direction when a fluidpressure differential between a first chamber and a second chamberwithin the shock absorber is below a predetermined threshold. The methodalso includes providing a second delay force, less than the first. delayforce, when the fluid pressure differential is at, or above, thepredetermined threshold.

A preferred embodiment is a shock absorber including a first fluidchamber, a second fluid chamber and a fluid circuit connecting the firstfluid chamber and the second fluid chamber. An inertia valve includes aninertia mass and a moveable stop. The inertia mass is movable between anopen position and a closed position. The moveable stop is movablebetween a first position and a second position. The inertia mass isbiased to move toward the closed position at substantially a first rate.The moveable stop and the inertia mass cooperate to define a pocketconfigured to receive the other of the moveable stop and the inertiamass in the open position of the inertia mass and the first position ofthe moveable stop. The movement of the inertia mass toward the closedposition is restrained to a second rate less than the first rate. Themoveable stop moves from the first position to the second position inresponse to a pressure within the second fluid chamber being greaterthan a pressure within the first fluid chamber by at least apredetermined pressure differential threshold, thereby permitting theinertia mass to return to the closed position at substantially the firstrate.

A preferred embodiment is a damper including a first fluid chamber and asecond fluid chamber. A fluid circuit connects the first fluid chamberand the second fluid chamber. An acceleration sensor is configured toproduce a control signal in response to an acceleration force above afirst predetermined threshold. The damper also has an inertia valveincluding an inertia mass that at least partially comprises a magneticmaterial and is movable between a first position and a second position.The inertia valve permits a first rate of fluid flow through the fluidcircuit in the first position of the inertia mass and a second rate offluid flow through the fluid circuit in the second position of theinertia mass. The second rate of fluid flow is non-equal to the firstrate. The inertia mass moves in a direction from the first position tothe second position in response to an acceleration force above a secondpredetermined threshold. An electromagnetic force generator is capableof retaining the inertia mass in the second position. A control systemis configured to receive the control signal from the sensor andselectively activate the electromagnetic element in response to thecontrol signal to retain the inertia mass in the second position for apredetermined period of time after the acceleration force diminishesbelow the first predetermined threshold.

A preferred embodiment is a bicycle including a front wheel defining ahub axis, a rear wheel, and a main frame. An acceleration sensor ismounted for movement with the hub axis of the front wheel and isconfigured to produce a control signal in response to sensing anacceleration above a predetermined threshold. A shock absorber isoperably positioned between the rear wheel and the frame. The shockabsorber includes a valve arrangement configured to receive the controlsignal from the sensor and to selectively alter a damping rate of theshock absorber in response to the control signal.

A preferred embodiment is a bicycle including a front wheel defining ahub axis, a rear wheel, and a main frame. An acceleration sensor ismounted for movement with the hub axis of the front wheel and isconfigured to produce a control signal in response to sensing anacceleration above a predetermined threshold. A shock absorber isoperably positioned between the front wheel and the frame and includes avalve arrangement configured to receive the control signal from thesensor. The valve arrangement is configured to selectively alter adamping rate of the shock absorber in response to the control signal.

A preferred embodiment is a method of altering a rate of damping of abicycle rear wheel shock absorber including sensing an accelerationforce above a predetermined threshold acting on a hub axis of a frontwheel of said bicycle. The method further includes providing a valveassembly within said rear wheel shock absorber configured to selectivelyalter a damping rate of said rear wheel shock absorber, and alteringsaid damping rate of said rear wheel shock absorber in response to anacceleration force above said predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the damper will now be described withreference to drawings of preferred embodiments. The embodiments areillustrated in the context of use on an off-road bicycle, however, theseembodiments are merely intended to illustrate, rather than limit, thepresent invention. The drawings contain the following figures:

FIG. 1 is a perspective view of a bicycle including preferred front andrear shock absorbers;

FIG. 2 is a cross-section of the rear shock absorber of FIG. 1;

FIG. 3 a is an enlarged cross-section of a main portion of the shockabsorber of FIG. 2 and FIG. 3 b is an enlarged cross-section of areservoir of the shock absorber of FIG. 2 showing an inertia valve in aclosed position;

FIG. 4 a is a top plan view of the inertia mass of the shock absorber ofFIG. 2. FIG. 4 b is a side cross-section view of the inertia mass ofFIG. 2 taken along line 4 b-4 b in FIG. 4 a. FIG. 4 c is a bottom planview of the inertia mass of FIG. 2;

FIG. 5 is an enlarged cross-section of the reservoir of the shockabsorber of FIG. 2, showing the inertia valve in an open position;

FIG. 6 is an enlarged cross-section of the inertia valve of the shockabsorber of FIG. 2;

FIG. 7 a is an enlarged view of a portion of the inertia valve of FIG.6. FIG. 7 b is an enlarged view of a portion of an alternative inertiavalve;

FIG. 8 is a graph illustrating the relationship between position,velocity and acceleration for a simple mass;

FIG. 9 is a schematic illustration of an inertia valve in an off-centercondition;

FIG. 10 is a schematic illustration of an inertia valve in a secondoff-center condition;

FIG. 11 is a cross-section view of the inertia valve of FIG. 3 b showingvarious zones of cross-sectional fluid flow areas;

FIG. 12 is a cross-section view of the inertia valve of FIG. 3 b in anoff-center condition;

FIG. 13 is an enlarged view of an adjustable return fluid flow beneaththe inertia mass;

FIG. 14 is the front shock absorber, or suspension fork, of FIG. 1 asdetached from the bicycle;

FIG. 15 is a cross-section view of the right leg of the fork of FIG. 14,illustrating various internal components;

FIG. 16 is an enlarged cross-section of a lower portion of the fork legof FIG. 15, illustrating an inertia valve damping system;

FIG. 17 is an enlarged cross-section of a base valve assembly of thelower portion of the fork leg of FIG. 16;

FIG. 18 is a cross-section view of the lower portion of the fork of FIG.15, with the inertia valve in an open position;

FIG. 19 is the base valve assembly of FIG. 17, with the inertia valve inan open position;

FIG. 20 is a cross-section view of the lower portion of the fork of FIG.16 illustrating rebound fluid flow;

FIG. 21 is the base valve assembly of FIG. 17 illustrating rebound fluidflow;

FIG. 22 is a cross-section view of a lower portion of an alternativeembodiment of a suspension fork;

FIG. 23 is an enlarged view of the base valve assembly of the fork ofFIG. 22, with the inertia valve in a closed position;

FIG. 24 is the lower portion of the fork of FIG. 22, with the inertiavalve in an open position;

FIG. 25 is the base valve assembly of FIG. 23, with the inertia valve inan open position;

FIG. 26 is a graph of the pressure differential of fluid acting on theleft and right sides of the inertia mass versus internal diameter of theinertia mass;

FIG. 27 is a graph of the pressure differential factor of fluid actingon the left and right sides of the inertia mass versus the internaldiameter of the inertia mass for a radial gap between the inertia massand shaft of 0.002 inches; and

FIG. 28 is a graph of the pressure differential factor of fluid actingon the left and right sides of the inertia mass versus the internaldiameter of the inertia mass for a radial gap between the inertia massand shaft of 0.001 inches.

FIG. 29 is an enlarged, cross-section view of an alternative inertiavalve assembly comprising an inertia mass having increased density, incomparison to the embodiments of FIGS. 1-28, in order to provideincreased responsiveness to acceleration forces.

FIG. 30 is an enlarged view of an alternative embodiment of an inertiamass including a plurality of drag members to increase the fluid drag onthe inertia mass when moving in one direction in comparison with thedrag on the inertia mass during movement in the opposite direction.

FIG. 31A is a cross-section view of the inertia mass of FIG. 30illustrating an orientation of the drag members when the inertia mass ismoving in a downward direction within a fluid-filled reservoir chamber.FIG. 31B is a cross-section view of the inertia mass of FIG. 30illustrating an orientation of the drag members when the inertia mass ismoving in an upward direction within a fluid-filled reservoir chamber.

FIG. 32 is an enlarged, cross-section view of a pressure-responsiveinertia valve assembly.

FIG. 33 is an enlarged, cross-section view of another embodiment of apressure-responsive inertia valve assembly.

FIG. 34 is a side elevational view of bicycle employing yet anotherembodiment of an acceleration-sensitive shock absorber.

FIG. 35 is an enlarged, cross-section view of an acceleration-sensitivevalve assembly within a shock absorber of the bicycle of FIG. 35. Theinertia valve assembly of FIG. 30 includes a valve body that is at leastpartially controlled by an electro-magnetic system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an off-road bicycle, or mountain bike, 20 including aframe 22 which is comprised of a main frame portion 24 and a swing armportion 26. The swing arm portion 26 is pivotally attached to the mainframe portion 24. The bicycle 20 includes front and rear wheels 28, 30connected to the main frame 24. A seat 32 is connected to the main frame24 and provides support for a rider of the bicycle 20.

The front wheel 28 is supported by a preferred embodiment of asuspension fork 34 which, in turn, is secured to the main frame 24 by ahandlebar assembly 36. The rear wheel 30 is connected to the swing armportion 26 of the frame 22. A preferred embodiment of a rear shock 38 isoperably positioned between the swing arm 26 and the main frame 24 toprovide resistance to the pivoting motion of the swing arm 26. Thus, theillustrated bicycle 20 includes suspension members 34, 38 between thefront and rear wheels 28, 30 and the frame 22, which operate tosubstantially reduce wheel impact forces from being transmitted to therider of the bicycle 20. The rear shock absorber 38 desirably includes afluid reservoir 44 hydraulically connected to the main shock body by ahydraulic hose 46. Preferably, the reservoir 44 is connected to theswingarm portion 26 of the bicycle 20 above the hub axis of the rearwheel 30.

The suspension fork 34 and the rear shock 38 preferably include anacceleration-sensitive valve, commonly referred to as an inertia valve,which allows the damping rate to be varied depending upon the directionof an acceleration input. The inertia valve permits the suspension fork34 and rear shock 38 to distinguish between accelerations originating atthe sprung mass, or main frame 24 and rider of the bicycle 20, fromaccelerations originating at the unsprung mass, or front wheel 28 andrear wheel 30, and alter the damping rate accordingly. It is generallypreferred to have a firm damping rate when accelerations originate atthe sprung mass and a softer damping rate when the accelerationsoriginate at the unsprung mass. On an automobile or other four-wheelvehicle, this helps to stabilize the body by reducing fore and aftpitching motions during acceleration and braking, as well as by reducingbody roll during cornering.

In a similar manner, on two-wheel vehicles such as motorcycles andbicycles, vehicle stability is improved by reduction of fore and aftpitching motions. In addition, in the case of bicycles and otherpedal-driven vehicles, this reduces or prevents suspension movement inresponse to rider-induced forces, such as pedaling forces, whileallowing the suspension to absorb forces induced by the terrain on whichthe bicycle 20 is being ridden. As will be described in detail below,the inertia valving within the suspension fork 34 and rear shock 38include features which permit responsive, consistent performance andallow such inertia valves to be manufactured in a cost effective manner.Preferably, the inertia valve is located within the reservoir 44, whichmay be rotated relative to the swingarm portion 26 of the bicycle 20.Rotating the reservoir 44 alters the component of an upward accelerationof the rear wheel 30 which acts along the axis of motion of the inertiavalve and thereby influences the responsiveness of the inertia valve.

FIGS. 2-7 illustrate a preferred embodiment of the rear shock absorber38. A shock absorber 38 operates as both a suspension spring and as adamper. Preferably, the spring is an air spring arrangement, but coilsprings and other suitable arrangements may also be used. The shock 38is primarily comprised of an air sleeve 40, a shock body 42 and areservoir 44. In the illustrated embodiment, a hydraulic hose 46physically connects the main body of the shock 38 (air sleeve 40 andshock body 42) to the reservoir 44. However, the reservoir 44 may alsobe directly connected to the main body of the shock absorber 38, such asbeing integrally connected to, or monolithically formed with, the airsleeve 40.

The air sleeve 40 is cylindrical in shape and includes an open end 48and an end closed by a cap 50. The cap 50 of the air sleeve 40 definesan eyelet 52 which is used for connection to the main frame 24 of thebicycle 20 of FIG. 1. The open end 48 of the air sleeve 40 slidinglyreceives the shock body 42.

The shock body 42 is also cylindrical in shape and includes an open end54 and a closed end 56. The closed end 56 defines an eyelet 58 forconnecting the shock 38 to the swing arm portion 26 of the bicycle 20 ofFIG. 1. Thus, the air sleeve 40 and the shock body 42 are configured fortelescopic movement between the main frame portion 24 and the swing armportion 26 of the bicycle 20. If desired, this arrangement may bereversed and the shock body 42 may be connected to the main frame 24while the air sleeve 40 is connected to the swing arm 26.

A seal assembly 60 is positioned at the open end 48 of the air sleeve 40to provide a substantially airtight seal between the air sleeve 40 andthe shock body 42. The seal assembly 60 comprises a body seal 62positioned between a pair of body bearings 64. The illustrated body seal62 is an annular seal having a substantially square cross-section.However, other suitable types of seals may also be used. A wiper 66 ispositioned adjacent the open end 48 of the air sleeve 40 to removeforeign material from the outer surface of the shock body 42 as it movesinto the air sleeve 40.

A damper piston 68 is positioned in sliding engagement with the innersurface of the shock body 42. A shock shaft 70 connects the piston 68 tothe cap 50 of the air sleeve 40. Thus, the damper piston 68 is fixed formotion with the air sleeve 40.

A piston cap 72 is fixed to the open end 54 of the shock body 42 and isin sliding engagement with both the shock shaft 70 and the inner surfaceof the air sleeve 40. The piston cap 72 supports a seal assembly 74comprised of a seal member 76 positioned between a pair of bearings 78.The seal assembly 74 is in a sealed, sliding engagement with the innersurface of the air sleeve 40. A shaft seal arrangement 80 is positionedto create a seal between the cap 72 and the shock shaft 70. The shaftseal arrangement 80 comprises a seal member 82 and a bushing 84. Theseal member 82 is an annular seal with a substantially squarecross-section, similar to the body seal 62. The shaft seal arrangement80 creates a substantially airtight seal between the cap 72 and theshock shaft 70 while allowing relative sliding motion therebetween.

A positive air chamber 86 is defined between the closed end 50 of theair sleeve 40 in the cap 72. Air held within the positive air chamber 86exerts a biasing force to resist compression motion of the shockabsorber 38. Compression motion of the shock absorber 38 occurs when theclosed ends 56 and 50 of the shock body 42 and air sleeve 40 (and thusthe eyelets 52, 58) move closer to one another.

A negative air chamber 88 is defined between the cap 72 and the sealassembly 60, which in combination with the shock body 42 closes the openend 48 of the air sleeve 40. Air trapped within the negative air chamber88 exerts a force which resists expansion, or rebound, motion of theshock absorber 38. Rebound motion of the shock absorber 38 occurs whenthe closed ends 56 and 50 of the shock body 42 and air sleeve 40 (andthus the eyelets 52, 58) move farther apart from each other. Together,the positive air chamber 86 and the negative air chamber 88 function asthe suspension spring portion of the shock absorber 38.

An air valve 90 communicates with the positive air chamber 86 to allowthe air pressure therein to be adjusted. In this manner, the spring rateof the shock absorber 38 may be easily adjusted.

A bypass valve 92 is provided to allow the pressure between the positiveair chamber 86 and the negative air chamber 88 to be equalized. Thebypass valve 92 is configured to allow brief communication between thepositive air chamber 86 and the negative air chamber 88 when the airsleeve seal assembly 74 passes thereby. A bottom out bumper 94 ispositioned near the closed end 50 of the air sleeve 40 to prevent directmetal to metal contact between the closed end 50 and the cap 72 of theshock body 42 upon full compression of the shock absorber 38.

The shock absorber 38 also includes a damper assembly, which is arrangedto provide a resistive force to both compression and rebound motion ofthe shock absorber 38. Preferably, the shock absorber 38 provides modalresponse compression damping. That is, the shock absorber 38 preferablyoperates at a first damping rate until an appropriate acceleration inputis sensed, then the shock absorber 38 operates at a second damping ratefor a predetermined period thereafter, before returning the firstdamping rate. This is in opposition to a system that attempts tocontinually respond to instantaneous input. Such a modal system avoidsthe inherent delay associated with responding separately to each inputevent.

The piston 68 divides the interior chamber of the shock body 42 into acompression chamber 96 and a rebound chamber 98. The compression chamber96 is defined between the piston 68 and the closed end 56 of the shockbody 42 and decreases in volume during compression motion of the shockabsorber 38. The rebound chamber 98 is defined between the piston 68 andthe piston cap 72, which is fixed to the open end 54 of the shock body42. The rebound chamber 98 decreases in volume upon rebound motion ofthe shock absorber 38.

The piston 68 is fixed to the shock shaft 70 by a hollow threadedfastener 100. A seal 102 is fixed for movement with the piston 68 andcreates a seal with the inner surface of the shock body 42. Theillustrated seal 102 is of an annular type having a rectangularcross-section. However, other suitable types of seals may also be used.

The piston 68 includes one or more axial compression passages 104 thatare covered on the rebound chamber 98 side by a shim stack 106. As isknown, the shim stack 106 is made up of one or more flexible shims anddeflects to allow flow through the compression passages 104 duringcompression motion of the shock absorber 38 but prevents flow throughthe compression passages 104 upon rebound motion of the shock absorber38. Similarly, the piston 68 includes one or more rebound passages 108extending axially therethrough. A rebound shim stack 110 is made up ofone or more flexible shims, and deflects to allow flow through therebound passages 108 upon rebound motion of the shock absorber 38 whilepreventing flow through the rebound passages 108 during compressionmotion of the shock absorber 38.

A central passage 112 of the shock shaft 70 communicates with thecompression chamber 96 through the hollow fastener 100. The passage 112also communicates with the interior chamber of the reservoir 44 througha passage 114 defined by the hydraulic hose 46. Thus, the flow ofhydraulic fluid is selectively permitted between the compression chamber96 and the reservoir 44.

A rebound adjustment rod 116 extends from the closed end 50 of the airsleeve 40 and is positioned concentrically within the passage 112 of theshock shaft 70. The rebound adjustment rod 116 is configured to alterthe amount of fluid flow upon rebound motion thereby altering thedamping force produced. An adjustment knob 118 engages the reboundadjustment rod 116 and is accessible externally of the shock absorber 38to allow a user to adjust the rebound damping rate. A ball detentmechanism 120 operates in a known manner to provide distinct adjustmentpositions of the rebound damping rate.

The reservoir 44 includes a reservoir tube 122 closed on either end. Afloating piston 124 is in sliding engagement with the interior surfaceof a reservoir tube 122. A seal member 126 provides a substantiallyfluid-tight seal between the piston 124 and the interior surface of thereservoir tube 122. The seal member 126 is preferably an annular sealhaving a substantially square cross-section. However, other suitableseals may also be used.

The floating piston 124 divides the interior chamber of the reservoirtube 122 into a reservoir chamber 128 and a gas chamber 130. Thereservoir chamber 128 portion of the reservoir tube is closed by an endcap 132. The end cap 132 additionally receives the end of the hydraulichose 46 and supports a hollow reservoir shaft 134. The central passage136 of the reservoir shaft 134 is in fluid communication with thepassages 114 and 112 and, ultimately, the compression chamber 96.

The reservoir shaft 134 supports an inertia valve assembly 138 and ablowoff valve assembly 140. Each of the inertia valve assembly 138 andthe blowoff valve assembly 140 allows selective communication betweenthe compression chamber 96, via the passages 112, 114, 136, and thereservoir chamber 128.

The gas chamber 130 end of the reservoir tube 122 is closed by a cap 142which includes a valve assembly 144 for allowing gas, such as nitrogen,for example, to be added or removed from the gas chamber 130. Thepressurized gas within the gas chamber 130 causes the floating piston124 to exert a pressure on the hydraulic fluid within the reservoirchamber 128. This arrangement prevents air from being drawn into thehydraulic fluid and assists in refilling fluid into the compressionchamber 96 during rebound motion of the shock absorber 38.

With reference to FIG. 3 b, the blowoff valve assembly 140 is supportedby the reservoir shaft 134 and positioned above the inertia valveassembly 138. The reservoir shaft 134 reduces in diameter to define ashoulder portion 154. An annular washer 156 is supported by the shoulder154 and the blowoff valve assembly 140 is supported by the washer 156.The washer 156 also prevents direct contact between the inertia mass 150and the blowoff valve assembly 140.

The blowoff valve assembly 140 is primarily comprised of a cylindricalbase 158 and the blowoff cap 160. The base 158 is sealed to thereservoir shaft 134 by a shaft seal 162. The illustrated seal 162 is anO-ring, however other suitable seals may also be used. The upper end ofthe base 158 is open and includes a counterbore which defines a shoulder164. The blowoff cap 160 is supported by the shoulder 164 and is sealedto the inner surface of the base 158 by a cap seal 166. The cap seal 166is preferably an O-ring, however other suitable seals may also be used.A threaded fastener 168 fixes the blowoff cap 160 and base 158 to thereservoir shaft 134.

The blowoff cap 160 and base 158 define a blowoff chamber 170therebetween. A plurality of radial fluid flow passages 172 are definedby the reservoir shaft 134 to allow fluid communication between theblowoff chamber 170 and the shaft passage 136.

The blowoff cap 160 includes one or more axial blowoff passages 174 andone or more axial refill passages 176. A blowoff shim stack 178 ispositioned above the blowoff cap 160 and covers the blowoff passages174. The blowoff shim stack 178 is secured in place by the threadedfastener 168. The individual shims of the shim stack 178 are capable ofdeflecting about the central axis of the fastener 168 to selectivelyopen the blowoff passages 174 and allow fluid communication between theblowoff chamber 170 and the reservoir chamber 128. The blowoff shimstack 178 is preferably configured to open in response to pressureswithin the blowoff chamber above a minimum threshold, such asapproximately 800 psi, for example.

A refill shim stack 180 is positioned between the blowoff cap 160 andthe reservoir shaft 134 and covers the refill ports 176. The refill shimstack 180 is configured to prevent fluid from flowing from the blowoffchamber 170 through ports 176 to the reservoir 128 while offering littleresistance to flow from the reservoir 128 into the blowoff chamber 170.

The inertia valve assembly 138 includes a plurality of radiallyextending, generally cylindrical valve passages 148, connecting thepassage 136 to the reservoir chamber 128. The inertia valve assembly 138also includes a valve body, or inertia mass 150, and a spring 152. Thespring 152 biases the inertia mass 150 into an upward, or closed,position wherein the inertia mass 150 covers the mouths of the valvepassages 148 to substantially prevent fluid flow from the passage 136 tothe reservoir chamber 128. The inertia mass 150 is also movable into adownward, or open, position against the biasing force of the spring 152.In the open position, the inertia mass 150 uncovers at least some of thevalve passages 148 to allow fluid to flow therethrough.

The end cap 132, which closes the lower end of the reservoir tube 122,defines a cylindrical pocket, or socket, 182 which receives the inertiamass 150 in its lowermost or open position. The lowermost portion of thepocket 182 reduces in diameter to form a shoulder 184. The shoulder 184operates as the lowermost stop surface, which defines the open positionof the inertia mass 150, as illustrated in FIG. 5.

The inertia mass 150 includes a check plate 190 which allows fluid to bequickly displaced from the pocket 182 as the inertia mass 150 movesdownward into the pocket 182. The inertia mass 150 has a plurality ofaxial passages 188 extending therethrough. The check plate 190 rests onseveral projections, or standoff feet, 192 (FIG. 6) slightly above theupper surface of the inertia mass 150 and substantially covers thepassages 188. A series of stop projections 193, similar to the standofffeet, are formed or installed in the upper, necked portion of theinertia mass 150 to limit upward motion of the check plate 190.

With reference to FIG. 4 a, a top plan view of the inertia mass 150 isshown. The axial passages 188 are preferably kidney-shaped, to allow thepassages 188 to occupy a large portion of the transverse cross-sectionalarea of the inertia mass 150. Desirably, the ratio of the passage 188cross-sectional area to the inertia mass 150 cross-sectional area isgreater than approximately 0.3. Preferably, the ratio of the passage 188cross-sectional area to the inertia mass 150 cross-sectional area isgreater than approximately 0.5, and more preferably greater thanapproximately 0.7.

The large area of the passages 188 provides a low-resistance flow pathfor hydraulic fluid exiting the pocket 182. As a result, the flow rateof the fluid exiting the pocket 182 is high, and the inertia mass isable to move rapidly into the open position. In addition, the amount offluid which must be displaced by the inertia mass 188 for it to moveinto the open position is reduced. Advantageously, such an arrangementallows the inertia mass 150 to respond rapidly to acceleration forces.

When the check plate 190 is resting against the standoff feet 192 on theupper surface of the inertia mass 150 it provides restricted fluid flowthrough the passages 188. The check plate 190 also has an open positionin which it moves upward relative to the inertia mass 150 until itcontacts the stop projections 193. When the check plate 190 is open,fluid is able to flow from the pocket 182 through the passages 188 andinto the reservoir 128, with desirably low resistance.

The inertia mass 150 also includes a third series of projections, orstandoff feet, 194. The standoff feet 194 are comprised of one or moreprojections located on the uppermost surface of the upper neck portionof the inertia mass 150. The standoff feet 194 on the upper surface ofthe neck portion of the inertia mass 150 contact the washer 156 when theinertia mass 150 is in its uppermost or closed position. A fourth set ofprojections, or standoff feet, 195 are positioned on the lower surfaceof the inertia mass 150 (FIG. 4 c) and contact the shoulder 184 when theinertia mass 150 is in its lower or open position.

In each set of stop projections, or standoff feet, 192-195, preferablybetween three to five individual projections are disposed radially aboutthe inertia mass 150. However, other suitable numbers of feet may alsobe used. Desirably, the surface area of the stop projections, orstandoff feet, 192-195 is relatively small. A small surface area of thestandoff feet 194, 195 lowers the resistance to movement of the inertiamass 150 by reducing the overall surface contact area between theinertia mass 150 and the washer 156 or shoulder 184, respectively. Thesmall surface area of the standoff feet 192 and stop projections 193lower the resistance to movement of the check plate 190 relative to theinertia mass 150. Desirably, the projections 192-195 have dimensions ofless than approximately 0.025″×0.025″. Preferably, the projections192-195 have dimensions of less than approximately 0.020″×0.020″ and,more preferably, the projections 192-195 have dimensions of less thanapproximately 0.015″×0.015″.

When utilized with an inertia mass 150 having a mass (weight) ofapproximately 0.5 ounces, the preferred projections 192-195 provide adesirable ratio of the mass (weight) of the inertia valve mass 150 tothe contact surface area of the projections 192-195. Due to the vacuumeffect between two surfaces, a force of approximately 14.7 lbs/in²(i.e., atmospheric pressure) is created when attempting to separate theinertia mass 150 from either the washer 156 or shoulder 184,respectively. By lowering the contact surface area between the inertiamass 150 and either the washer 156 or shoulder 184, the vacuum forcetending to resist separation of the contact surfaces is desirablyreduced.

Preferably, the contact surface area is small in comparison with themass (weight) of the inertia mass 150 because the magnitude of theacceleration force acting on the inertia mass 150 is proportional toit's mass (weight). Accordingly, a large ratio of the mass (weight) ofthe inertia valve mass 150 to the contact surface area of theprojections 192-195 is desired. For example, for a set of three (3)standoff feet 194, 195 with dimensions of approximately 0.025″×0.025″,the ratio is at least approximately 17 lbs/in². A more desirable ratiois at least approximately 25 lbs/in², Preferably, the ratio is at least50 lbs/in² and more preferably is at least 75 lbs/in². These ratios aredesirable for an inertia mass utilized in the context of an off-roadbicycle rear shock absorber and other ratios may be desirable for otherapplications and/or vehicles. Generally, however, higher ratios increasethe sensitivity of the inertia mass 150 (i.e., allow the inertia mass150 to be very responsive to acceleration forces). For example, with aratio of 50 lbs/in² the sensitivity of the inertia mass 150 is about+/−⅓ G. Likewise, for a ratio of 147 lbs/in² the sensitivity of theinertia mass 150 is about +/− 1/10 G.

As illustrated in FIG. 6, the outside diameter of the lower portion ofthe inertia mass 150 is slightly smaller than the diameter of the pocket182. Therefore, an annular clearance space is defined between them whenthe inertia mass 150 is positioned within the pocket 182. The clearanceC restricts the rate with which fluid may pass to fill the pocket belowthe inertia mass 150, to influence the rate at which the inertia mass150 may exit the pocket 182.

The interior surface of the inertia mass 150 includes an increaseddiameter central portion 195 which, together with the shaft 134, definesan annular recess 196. The annular recess 196 is preferably locatedadjacent to one or more of the ports 148 when the inertia mass 150 is inits closed position. Thus, fluid exiting from the shaft passage 136through the passages 148 enters the annular recess 196 when the inertiamass 150 is its closed position.

The interior surface of the inertia mass 150 decreases in diameter bothabove and below the central portion 195 to create an upper intermediateportion 197 and a lower intermediate portion 199. The upper intermediateportion 197 and lower intermediate portion 199, together with the shaft134, define an upper annular clearance 198 (FIG. 7 a) and a lowerannular clearance 200, respectively. An upper lip 201 (FIG. 7 a) ispositioned above, and is of smaller diameter than, the upperintermediate portion 197. A step 205 (FIG. 7 a) is defined by thetransition between the upper intermediate portion 197 and the upper lip201. Similarly, a lower lip 203 is positioned below, and has a smallerdiameter than, the lower intermediate portion 199. A step 205 is definedby the transition between the lower intermediate portion 199 and thelower lip 203. The upper lip 201 and the lower lip 203, together withthe shaft 134, define an upper exit clearance 202 (FIG. 7 a) and a lowerexit clearance 204.

With reference to FIG. 7 a, the upper lip 201 preferably includes alabyrinth seal arrangement 206. As is known, a labyrinth seal comprisesa series of annular grooves formed into a sealing surface, Preferably,the lower lip 203 also includes a labyrinth seal arrangementsubstantially similar to the labyrinth seal 206 of the upper lip 201.

Advantageously, the labyrinth seal arrangement 206 reduces fluid flow(bleed flow) between the reservoir shaft 134 and the upper lip 201 whenthe inertia mass 150 is in a closed position. Excessive bleed flow isundesired because it reduces the damping rate when the inertia valve 138is closed. By utilizing a labyrinth seal 206, the clearance between theinertia mass 150 and the shaft 134 may be increased, without permittingexcessive bleed flow. The increased clearance is particularly beneficialto prevent foreign matter from becoming trapped between the inertia mass150 and shaft 134 and thereby inhibiting operation of the inertia valve138. Thus, reliability of the shock absorber 38 is increased, while theneed for routine maintenance, such as changing of the hydraulic fluid,is decreased.

With reference to FIG. 7 b, an alternative inertia mass 150 isillustrated. The upper intermediate portion 197 of the inner surface ofthe inertia mass 150 of FIG. 7 b is inclined with respect to the outersurface of the shaft 134, rather than being substantially parallel tothe outer surface of the shaft 134 as in the inertia mass of FIG. 7 a.Thus, in the inertia mass 150 of FIG. 7 b, the step 205 is effectivelydefined by the entire upper intermediate portion 197. The inertia mass150 configuration of FIG. 7 b theoretically provides approximatelyone-half the self-centering force of the inertia mass 150 of FIG. 7 a.In addition, other suitable configurations of the inner surface of theinertia mass 150 may be utilized to provide a suitable self-centeringforce, as will be apparent to one of skill in the art based on thedisclosure herein. For example, the inclined surface may begin in anintermediate point of the upper intermediate portion 197. Altematively,the step 205 may be chamfered, rather than orthogonal.

With reference to FIGS. 1-7, the operation of the shock absorber 38 willnow be described in detail. As described previously, the shock absorber38 is operably mounted between the main frame 24 and the swing armportion 26 of the bicycle 20 and is capable of both compression andrebound motion. Preferably, the shock body 42 portion of the shockabsorber 38 is connected to the swing arm portion 26 and the air sleeve40 is connected to the main frame 24. The reservoir 44 is desirablyconnected to the swing arm portion 26 of the bicycle 20 preferably nearthe rear axle, and preferably approximately vertical as shown in FIG. 1.

When the rear wheel 30 of the bicycle 20 encounters a bump the swing armportion 26 articulates with respect to the main frame 24, tending tocompress the shock absorber 38. If the acceleration imparted along thelongitudinal axis of the reservoir 44 is below a predeterminedthreshold, the inertia mass 150 will remain in its closed position, heldby the biasing force of the spring 152, as illustrated in FIG. 3 b.

For the piston 68 to move relative to the shock body 42 (i.e.,compression motion of the shock absorber 38) a volume of fluid equal tothe displaced volume of the shock shaft 70 must be transferred into thereservoir 128. With the inertia mass 150 closing the passages 148 andthe blowoff valve 140 remaining in a closed position, fluid flow intothe reservoir 128 is substantially impeded and the shock absorber 38remains substantially rigid.

If the compressive force exerted on the rear wheel 30, and thus theshock absorber 38, attains a level sufficient to raise the fluidpressure within the blowoff chamber 170 above a predetermined threshold,such as 800 psi for example, the blowoff shims 178 open to allow fluidto flow from the blowoff chamber 170 through the blowoff ports 174 andinto the reservoir 128. As an example, if the diameter of the shockshaft 70 is ⅝″ (Area=0.31 square inches) and the predetermined blow-offthreshold is 800 psi, then a compressive force at the shaft of at least248 pounds is required to overcome the blowoff threshold and commencecompression of the shock absorber. This required force, of course, is inaddition to the forces required, as is known in the art, to overcome thebasic spring force and the compression damping forces generated at thepiston 68 of the shock absorber. In this situation, compression of theshock absorber is allowed against the spring force produced by thecombination of the positive and negative air chambers 86, 88. Thedamping rate is determined by the flow through the compression ports 104of the piston 68 against the biasing force of the compression shim stack106. When the pressure within the blowoff chamber 170 falls below thepredetermined threshold, the blowoff shim stack 178 closes the blowoffports 174 and the shock absorber 38 again becomes substantially rigid,assuming the inertia mass 150 remains in the closed position.

If the upward acceleration imposed along the longitudinal axis of thereservoir 44 (i.e., the axis of travel of the inertia mass 150) exceedsthe predetermined minimum threshold, the inertia mass 150, which tendsto remain at rest, will overcome the biasing force of the spring 152 asthe reservoir 44 moves upward relative to the inertia mass 150. If theupward distance of travel of the reservoir 44 is sufficient, the inertiamass will move into the pocket 182. With the inertia mass 150 in theopen position, fluid is able to be displaced from the compressionchamber 96 through the passages 112, 114 and the shaft passage 136,through the passages 148 and into the reservoir 128. Thus, the shock 38is able to compress with the compression damping force again beingdetermined by flow through the compression ports 104 of the piston 68.

The predetermined minimum threshold for the inertia mass 150 to overcomethe biasing force of the spring 152 is determined primarily by the massof the inertia mass 150, the spring rate of the spring 152 and thepreload on the spring 152. Desirably, the mass of the inertia mass isapproximately 0.5 ounces, However, for other applications, such as thefront suspension fork 34 or vehicles other than off-road bicycles, thedesired mass of the inertia mass 150 may vary.

The spring rate of the spring 152 and the preload on the spring 152 arepreferably selected such that the spring 152 biases the inertia mass 150into a closed position when no upward acceleration is imposed along thelongitudinal axis of the reservoir 44. However, in response to such anacceleration force the inertia mass 150 will desirably overcome thebiasing force of the spring 152 upon experiencing an acceleration whichis between 0.1 and 3 times the force of gravity (G's). Preferably, theinertia mass 150 will overcome the biasing force of the spring 152 uponexperiencing an acceleration which is between 0.25 and 1.5 G's and morepreferably upon experiencing an acceleration which is between 0.4 and0.7 G's. For certain riding conditions or other applications, such asthe front suspension fork 34, or other applications besides off-roadbicycles, however, the predetermined threshold may be varied from thevalues recited above.

The check plate 190 resting on the standoff feet 193 of the inertia mass150 allows fluid to be easily displaced upward from the pocket 182 andthus allows the inertia mass 150 to move into the pocket 182 with littleresistance. This permits the inertia mass 150 to be very responsive toacceleration inputs. As the inertia mass 150 moves into the pocket 182,fluid within the pocket 182 flows through the passages 188 and lifts thecheck plate 190 against the stop projections 193.

Once the inertia mass 150 is in its open position within the pocket 182,as illustrated in FIG. 5, the spring 152 exerts a biasing force on theinertia mass 150 tending to move it from the pocket 182. Fluid pressureabove the inertia mass 150 causes the check plate 190 to engage thestandoff feet 192 located on the upper surface of the inertia mass 150restricting flow through the ports 188. The height of the standoff feet192 which the check plate 190 rests on is typically 0.003″ to 0.008″above the exit surface of the passages 188 to provide an adequate levelof flow restriction upon upward movement of the inertia mass 150. Fluidmay be substantially prevented from flowing through the passages 188 andinto the pocket 182, except for a small amount of bleed flow between thecheckplate 190 and the upper surface of the inertia mass 150. However,the height of the standoff feet 192 may be altered to influence the flowrate of the bleed flow and thereby influence the timer feature of theinertia mass 150, as will be described below.

Fluid also enters the pocket 182 through the annular clearance, orprimary fluid flow path, C (FIG. 6) between the interior surface, orvalve seat, of the pocket 182 and the exterior surface of the inertiamass 150. Thus, the size of the clearance C also influences the rate atwhich fluid may enter the pocket 182 thereby allowing the inertia mass150 to move upward out of the pocket 182.

Advantageously, with such a construction, once the inertia mass 150 ismoved into an open position within the pocket 182, it remains open for apredetermined period of time in which it takes fluid to refill thepocket behind the inertia mass 150 through the clearance C. This isreferred to as the “timer feature” of the inertia valve assembly 138.Importantly, this period of time can be independent of fluid flowdirection within the shock absorber 38. Thus, the shock absorber 38 mayobtain the benefits of a reduced compression damping rate throughout aseries of compression and rebound cycles, referred to above as “modalresponse.” Desirably, the inertia mass 150 remains in an open positionfor a period between approximately 0.05 and 5 seconds, assuming nosubsequent activating accelerations are encountered. Preferably, theinertia mass 150 remains in an open position for a period between about0.1 and 2.5 seconds and more preferably for a period between about 0.2and 1.5 seconds, again, assuming no subsequent accelerations areencountered which would tend to open the inertia mass 150, thuslengthening or resetting the timer period. The above values aredesirable for a rear shock absorber 38 for an off-road bicycle 20. Therecited values may vary in other applications, however, such as whenadapted for use in the front suspension fork 34 or for use in othervehicles or non-vehicular applications.

In order to fully appreciate the advantages of the modal responseinertia valve assembly 138 of the present shock absorber 38, it isnecessary to understand the operation of a bicycle having anacceleration-sensitive damping system utilizing an inertia valve. Withreference to FIG. 8, the relationship between vertical position P,vertical velocity V and vertical acceleration A, over time T, for asimple mass traversing two sinusoidally-shaped bumps is illustrated.FIG. 8 is based on a mass that travels horizontally at a constantvelocity, while tracking vertically with the terrain contour. Thisphysical model, somewhat simplified for clarity, correctly representsthe essential arrangement utilized in inertia-valve shock absorberswherein the inertial element is directly connected to, and driven by,the unsprung mass.

The primary simplification inherent in this model, and in this analysis,is that the flexibility of an actual bicycle tire is ignored. The tireis assumed to be inflexible in its interaction with the terrain,offering no compliance. An actual tire, of course, will provide somecompliance, which in turn produces some degree of influence on theposition, velocity, and acceleration of the unsprung mass. The actualdegree of influence in a given situation will depend on many variables,including the actual vehicle speed and the specific bump geometry, aswell as the compliance parameters of the particular tire. However, thesimplified analysis discussed here is a good first approximation whichclearly illustrates the key operative physics principles, while avoidingthese complications. The basic validity of this simplified analysis canbe demonstrated by a sophisticated computer motion analysis thatincorporates the effects of tire compliance and several othercomplicating factors.

Relating FIG. 8 to the situation of a bicycle, the heavy solid lineindicating position P represents both the trail surface and, assumingthe wheel of the bicycle is rigid and remains in contact with the trailsurface, the motion of any point on the unsprung portion of the bicycle,such as the hub axis of the front or rear wheel, for example. The linesrepresenting velocity V and acceleration A thus correspond to thevertical velocity and acceleration of the hub axis. In FIG. 8, the trailsurface (solid line indicating position P) includes a first bump B1 anda second bump B2. In this example, as shown, each bump is preceded by ashort section of smooth (flat) terrain.

As the wheel begins to traverse the first bump B1, the acceleration A ofthe hub axis H rises sharply to a maximum value and, accordingly, thevelocity V of the hub axis H increases. Mathematically, of course, theacceleration as shown is calculated as the second derivative of thesinusoidal bump curve, and the velocity as the first derivative. At apoint P1, approximately halfway up the first bump B1, the secondderivative (acceleration A) becomes negative (changes direction) and thevelocity begins to decrease from a maximum value. At a point P2,corresponding with the peak of the bump B1, the acceleration A is at aminimum value (i.e., large negative value) and the velocity V is atzero. At a point P3, corresponding with the mid-point of the downside ofthe first bump B1, the acceleration A has again changed direction andthe velocity V is at a minimum value (i.e., large negative value). At apoint P4, corresponding with the end of the first bump B1, theacceleration A has risen again to a momentary maximum value and thevelocity V is zero. The second bump B2 is assumed to besinusoidally-shaped like the first bump B1, but, as shown, to havesomewhat greater amplitude. Thus, the relationship between position P,velocity V and acceleration A are substantially identical to those ofthe first bump B1.

When a simple inertia valve is utilized in the suspension system of abicycle and the acceleration A reaches a threshold value, the inertiamass overcomes the biasing force of the spring and remains in asubstantially stationary position as the shaft which supports theinertia mass moves upward. Once the shaft has moved upward relative tothe inertia mass a sufficient distance, the inertia valve passages areuncovered and a reduced compression damping rate is achieved. Although acompression inertia valve is discussed in this example, the sameprinciples may be applied to an inertia valve which operates duringrebound.

Before the inertia valve passages are open, the shock absorber operatesat its initial, firm damping rate. This results in an undesirably firmdamping rate, creating a “damping spike”, over the initial portion ofthe bump B1. The damping spike continues until the shaft has movedupward relative to the inertia mass a sufficient distance to open thevalve passages. The amount of movement of the shaft relative to theinertia mass necessary to uncover the passages is determined primarilyby the size of the passages and the position of the uppermost surface ofthe inertia mass relative to the passages when the mass is in its fullyclosed position. This distance is referred to as the spike distanceS_(D). The amount of time necessary for the inertia passages to beopened and to reduce the damping rate is dependent upon the shape of thebump and the spike distance S_(D). and is referred to as the spike timeS_(T). The reduction of the damping rate is at least partially dependentupon the size of the passages and, therefore, it is difficult to reducethe spike time S_(T) without reducing the spike distance SD whichnecessarily affects the achievable lowered damping rate.

The inertia mass begins to close (i.e., move relatively upward) when theacceleration acting upon it either ceases, changes direction, or becomestoo small to overcome the biasing force of the spring. As showngraphically in FIG. 8, the acceleration A becomes zero at point P1, orat approximately the mid-point of the bump B1. Accordingly, a simpleinertia valve begins to close at, or before, the middle of the bump B1.Therefore, utilizing a simple inertia valve tends to return the shockabsorber to its initial, undesirably firm damping rate after only aboutone-half of the bump B1 has been traversed. The operating sequence ofthe inertia valve is similar for the second bump B2 and each bumpthereafter.

In actual practice, the specific point on a bump where a simple inertiavalve will close will vary depending on bump configuration, vehiclespeed, inertia valve size and geometry, spring bias force, compliance ofthe tire and other factors. Thus, it should be understood that theextent of mid-bump “spiking” produced by “premature closing” of a simpleinertia valve will be greater for some bumps and situations than forothers.

It is desirable to extend the amount of time the inertia valve staysopen so that the reduced damping rate can be utilized beyond the firsthalf of the bump. More complex inertia valve arrangements utilize thefluid flow during compression or rebound motion to hydraulically supportthe inertia valve in an open position once acceleration has ceased ordiminished below the level necessary for the inertia valve to remainopen from acceleration forces alone. However, these types of inertiavalve arrangements are dependent upon fluid flow and allow the inertiavalve to close when, or slightly before, the compression or reboundmotion ceases. A shock absorber using this type of inertia valve in thecompression circuit could experience a reduced damping rate from afterthe initial spike until compression motion ceases at, or near, the peakP2 of the bump B1. This would represent an improvement over the simpleinertia valve shock absorber described previously. However, the flowdependent inertia valve necessarily reacts to specific terrainconditions. That is, the inertia mass responds to each individualsurface condition and generally must be reactivated upon encounteringeach bump that the bicycle traverses. Therefore, this type of shockabsorber experiences an undesirably high damping rate “spike” as eachnew bump is encountered.

In contrast, the inertia valve arrangement 138 of the present shockabsorber 38 is a modal response type. That is, the inertia valve 138differentiates rough terrain conditions from smooth terrain conditionsand alters the damping rate accordingly. During smooth terrainconditions, the inertia valve 138 remains in a closed position and thedamping rate is desirably firm, thereby inhibiting suspension motion dueto the movement of the rider of the bicycle 20. When the first bump B1is encountered, the inertia valve 138 opens to advantageously lower thedamping rate so that the bump may be absorbed by the shock absorber 38.The timer feature retains the inertia valve 138 in an open position fora predetermined period of time thereby allowing the shock absorber 38 tomaintain the lowered damping rate for the entire bump (not just thefirst half), and to furthermore absorb the second bump B2 and subsequentbumps possibly without incurring any additional “spikes.” As discussedabove, the timer period may be adjustable by altering the rate at whichfluid may refill the timer pocket 182.

Once the shock absorber 38 has been compressed, either by fluid flowthrough the blowoff valve 140 or the inertia valve 138, the spring forcegenerated by the combination of the positive air chamber 86 and thenegative air chamber 88 tend to bias the shock body 42 away from the airsleeve 40. In order for the shock absorber 38 to rebound, a volume offluid equal to the displaced volume of the shock shaft 70 must be drawnfrom the reservoir 128 and into the compression chamber 96. Fluid flowis allowed in this direction through the refill ports 176 in the blowoffvalve 140 against a desirably light resistance offered by the refillshim stack 180. Gas pressure within the gas chamber 130 exerting a forceon the floating piston 124 may assist in this refill flow. Thus, therebound damping rate is determined primarily by fluid flow through therebound passages 108 against the biasing force of the rebound shim stack110.

With reference to FIGS. 3 b and 5, the fluid flow path duringcompression or rebound motion of the shock absorber 38, with the inertiamass 150 in either of an open or closed position, is above and away fromthe inertia mass 150 itself. Advantageously, such an arrangementsubstantially isolates fluid flow from coming into contact with theinertia mass 150, thereby inhibiting undesired movement of the inertiamass due to drag forces resulting from fluid flow. Thus, the inertiamass 150 advantageously responds to acceleration inputs and issubstantially unaffected by the movement of hydraulic fluid duringcompression or rebound of the shock absorber 38.

The present shock absorber 38 includes an inertia valve 138 comprising aself-centering valve body, or inertia mass 150. In order to fullyappreciate the advantages of the self-centering inertia mass 150 of thepresent inertia valve assembly 138, it is necessary to describe theconditions which have prevented prior inertia valve designs fromoperating reliably, with acceptable sensitivity, and for a reasonablecost.

Each of FIGS. 9 and 10 schematically illustrate an off-center conditionof the inertia mass 150 relative to the shaft 134. The off-centercondition of the inertia mass 150 may cause it to contact the shaft 134causing friction, which tends to impede motion of the inertia mass 150on the shaft 134. Due to the relatively small mass of the inertia mass150 and the desirability of having the inertia mass 150 respond to smallaccelerations, any friction between the inertia mass 150 and the shaft134 seriously impairs the performance of the inertia valve 138 and mayrender it entirely inoperable. Each of the off-center conditionsillustrated in FIGS. 8 and 9 may result from typical manufacturingprocesses. However, modifying the manufacturing process to avoid theseconditions often results in a prohibitively high manufacturing cost.

FIG. 9 illustrates an inertia valve arrangement in which the inertiavalve passages 148 are of slightly different diameter. Such a conditionis often an unavoidable result of the typical manufacturing process ofdrilling in a radial direction through a tubular piece of material. Sucha process may result in an entry diameter N created by the drilling toolbeing slightly larger than the exit diameter X created by the drillingtool. The resulting difference in area between the passages 148 causesthe fluid pressure within the shaft passage 136 to exert an unequalforce between the entry passage 148 having an entry diameter N and theexit passage 148 having an exit diameter X.

For example, a difference between the entry diameter N and the exitdiameter X of only two thousandths of an inch (0.090″ exit diameterversus 0.092″ entry diameter) at a fluid pressure of 800 psi, results ina force differential of approximately 0.2 pounds, or 3.6 ounces, betweenthe passages 148. The inertia mass 150 itself may weigh only about onehalf of an ounce (0.5 oz.). Such a force differential will push theinertia mass 150 off-center and reduce the responsiveness of the inertiamass 150, if not prevent it from moving entirely.

FIG. 10 illustrates an off-center condition of the inertia mass 150caused by the inertia valve passages 148 being positioned off-centerrelative to the shaft 134. A center axis AC of the inertia valvepassages 148 is offset from the desired diametrical axis AD of the shaft134 by a distance O. Therefore, the force resulting from fluid pressurewithin the shaft passage 136 does not act precisely on a diametricalaxis AD of the inertia mass 150, resulting in the inertia mass 150 beingpushed off-center with respect to, and likely contacting, the shaft 134.The offset condition of the center axis AC of the passages 148 is theresult of inherent manufacturing imperfections and cannot easily beentirely avoided, at least without raising the cost of manufacturing toan unfeasible level.

Furthermore, even if manufacturing costs were not of concern and thepassages 148 could be made with identical diameters and be positionedexactly along the diametrical axis AD of the shaft 134, additionalforces may tend to push the inertia mass 150 off-center. For example, ifthe reservoir 44 experiences an acceleration which is not exactlyaligned with the axis of travel of the inertia mass 150 (such as brakingor forward acceleration), the transverse component of the accelerationwould create a force tending to move the inertia mass 150 off-center andagainst the shaft 134. If the transverse component of the accelerationis large enough, the resulting frictional force between the inertia mass150 and the reservoir shaft 134 will inhibit, or prevent, movement ofthe inertia mass 150. Accordingly, it is highly desirable to compensatefor factors which tend to push the inertia mass 150 off-center in orderto ensure responsive action of the inertia valve 138. This is especiallyimportant in off-road bicycle applications, where it is desirable forthe inertia valve assembly 138 to respond to relatively smallaccelerations and the mass of the inertia mass 150 is also relativelysmall.

As described above, the inertia valve assembly 138 preferably includes aself-centering inertia mass 150. With reference to FIG. 11, the inertiamass 150 of FIG. 5 is shown without the fluid flow lines to more clearlydepict the cross-sectional shape of its interior surface. The inertiamass 150 has a minimum internal diameter “D” while the shaft 134 has aconstant external diameter “d,” which is smaller than the internaldiameter D. The difference between the shaft diameter d and the inertiavalve diameter D is desirably small. Otherwise, as described above, thebleed flow between the shaft 134 and the inertia mass 150 undesirablyreduces the damping rate which may be achieved when the inertia mass 150is in a closed position. Accordingly, for the rear shock 38 thedifference between the shaft diameter d and the inertia mass diameter Dis desirably less than 0.01 inches. Preferably, difference between theshaft diameter d and the inertia mass diameter D is less than 0.004inches and more preferably is approximately 0.002 inches. For the frontsuspension fork 34, the difference between the shaft diameter d and theinertia mass diameter D is desirably less than 0.02 inches. Preferably,difference between the shaft diameter d and the inertia mass diameter Dis less than 0.008 inches and more preferably is approximately 0.004inches. The recited values may vary in other applications, however, suchas when adapted for vehicles other than off-road bicycles ornon-vehicular applications.

The preferred differences between the shaft diameter d and the inertiamass diameter D recited above assume that a labyrinth seal arrangement206 (FIG. 7) is provided at the upper and lower portions of the internalsurface of the inertia mass 150, as described above. However, the bleedrate may be influenced by factors other than the difference between theshaft diameter d and the inertia mass diameter D. Accordingly, driven bya pressure differential of 400 psi, the bleed rate between the inertiamass 150 and the shaft 134, for an off-road bicycle shock with a shaftdiameter of ⅝ inches, is desirably less than 1.0 cubic inches/sec.Preferably, the bleed rate between the inertia mass 150 and the shaft134 is less than 0.5 cubic inches/sec and more preferably is less than0.3 cubic inches/sec. However, for applications other than off-roadbicycle shock absorbers, the preferred bleed rates may vary.

As described, an annular recess 196 is defined between the interiorsurface of the inertia mass 150 and the shaft 134. The annular recess196 is preferably located in approximately the center of the inertiamass 150. The annular recess 196 is referred to as zone 1 (Z₁) in thefollowing description of the fluid flow between the shaft 134 and theself-centering inertia mass 150. The upper annular clearance 198, abovethe annular recess 196, is referred to as zone 2 (Z₂) and the upper exitclearance 202 is referred to as zone 3 (Z₃). One half of the differencebetween the diameter of the upper annular clearance 198 and the diameterD at the upper exit clearance 202 defines a distance B, which isequivalent to the size of the step 205. The size B of the step 205(referred to as a “Bernoulli Step” in FIGS. 26, 27 and 28) may beprecisely manufactured by a computer controlled lathe operation, forexample. Other suitable methods for creating a precisely sized step 205may also be used.

Zone 1 Z₁ has a larger cross-sectional fluid flow area than zone 2 Z₂which, in turn, has a larger cross-sectional flow area than zone 3 Z₃.The cross-sectional area differential between the zones Z₁, Z₂, Z₃causes the fluid within each zone Z₁, Z₂, Z₃ to vary in velocity, whichcauses a self-centering force to be exerted on the inertia mass 150 whenit becomes off-center, as will be described below. Although the zonesZ₁, Z₂, Z₃ are annular, the discussion below is in the context of atwo-dimensional structure having left and right sides. Accordingly, thezones Z₁, Z₂, Z₃ of the example will vary in cross-sectional distance,rather than in cross-sectional area. Although the example is simplified,it correctly describes the general self-centering action of the inertiamass 150.

A rough approximation of the centering force developed by theself-centering inertia mass 150 can be estimated using Bernoulli'sequation. This is a rough approximation only since Bernoulli's equationassumes perfect frictionless flow, which is not valid for real fluids.However, this is a useful starting point for understanding the generalprinciples involved, and for estimating the forces that occur.Bernoulli's equation expresses the law of conservation of energy for theflow of an incompressible fluid. In estimating the centering force ofthe inertia mass 150, the potential energy (height) portion ofBernoulli's equation is not significant and may be ignored. Thus, forany two arbitrary points on a fluid streamline, Bernoulli's equationreduces to:P₁+(ρ/g)(V₁)²=P₂+(ρ/g)(V₂)²where:

-   -   P₁=fluid pressure (psi) at point 1    -   P₂=fluid pressure (psi) at point 2    -   V₁=fluid velocity (in/sec) at point 1    -   V₂=fluid velocity (in/sec) at point 2    -   ρ=fluid density    -   g=gravity constant

Using the values of 0.3125 lb/in³ for fluid density ρ of typicalhydraulic fluid and 386 in/sec² for gravity constant g, the equationbecomes:P₁+(4.05×10⁻⁵)(V₁)²=P₂+(4.05×10⁻⁵)(V₂)²

For a simple example, assume that the fluid pressure P₁ in zone 1 is 400psi, due to an external force tending to compress the shock absorber 38and the fluid velocity V₁ is zero due to relatively little fluid exitingfrom zone 1. Also, for simplicity, assume that the floating piston 124is absent or is not exerting a significant pressure on the fluid withinthe reservoir chamber 128. Accordingly, the fluid pressure P₃ in zone 3Z₃ is 0 psi. Insert these values into Bernoulli's equation to find thevelocity in zone 3:400+(4.05×10⁻⁵)(0)²=0+(4.05×10⁻⁵)(V₃)²V₃=3,142 in/sec

Therefore, as a first approximation (accurate to the degree that theassumptions Bernoulli's equation are based upon are valid here) thevelocity V₃ of fluid exiting zone 3 is 3,142 in/sec. Assuming thevalidity of assumptions inherent in Bernoulli's equation here, thisvalue is true for all exit points of zone 3 Z₃ regardless of theirdimensions. Further, based on flow continuity, the change in velocity ofthe fluid between zone 2 Z₂ and zone 3 Z₃ is proportional to the changein the clearance, or gap G, between zone 2 Z₂ and zone 3 Z₃. The gap Gis the cross-sectional distance between the outer surface of the shaft134 and the relevant inner surface of the inertia mass 150.

The relationship between the change in the size of the gap G and thechange in velocity allows solving of the velocity in zone 2 Z₂ for boththe right and left sides. Assuming that D is 0.379 inches, d is 0.375inches and B is 0.001 inches, then the gaps on both the right and leftsides, with the inertia mass 150 centered are:GAP Zone 2=B+(D−d)/2=0.003GAP Zone 3=(D−d)/2=0.002

Then, based on flow continuity, fluid velocity in Zone 2 is calculatedas follows:${V_{3}( \frac{{GAP}\quad{Zone}\quad 3}{{GAP}\quad{Zone}\quad 2} )} = {V_{2} = {2\text{,}094\quad{in}\text{/}\sec}}$

Therefore, the fluid velocity V₂ in zone 2 Z₂ for each of the right andleft side is 2,094 in/sec. Using Bernoulli's equation to find thepressure P₂ in zone two gives:400+(4.05×10⁻⁵)(0)²=(P₂)+(4.05×10⁻⁵)(2,094)²P₂=222 psi

Assuming that, for a particular inertia valve, the area in zone 2 Z₂that the fluid pressure acts upon for each of the right and left side is0.0375 in², then the force F at both the left and right sides of theinertia mass 150 can be calculated as:F=222 psi (0.0375 in²)=8.3 lbs.

The force F acting on the inertia mass 150 in the above example is equalfor the right and left side due to the velocity V₂ in zone 2 Z₂ beingthe same for each side. The velocity V₂ is the same because the ratio ofgap 3 G₃ to gap 2 G₂ between the right side and the left side is equaldue to the inertia mass 150 being centered relative to the shaft 134.

With reference to FIG. 12, however, if the inertia mass 150 becomes offcenter relative to the shaft 134 by a distance x, for example 0.001inches to the left, the ratio of gap 3 G₃ to gap 2 G₂ is differentbetween the right and left sides. This results in the velocity V₂ beingdifferent between the right and left sides and, as a result, a forcedifferential between the right side and left side is produced. Thesecalculations are substantially similar to the previous calculations andare provided below (for an off-center condition 0.001 inches to theleft:V₃=3,142 in/secLeft Side: GAP Zone 3(G_(3L))=(D−d)/2+x=0.003GAP Zone 2(G_(2L))=B+(D−d)/2+x=0.004${V_{3}( \frac{{GAP}\quad{Zone}\quad 3}{{GAP}\quad{Zone}\quad 2} )} = {V_{2} = {2356.5\quad{in}\text{/}\sec}}$ P₂=175 psiF=(175)(0.0375)=6.55 lbs.Right Side: GAP Zone 3(G_(3R))=(D−d)/2−x=0.001GAP Zone 2(G_(2R))=B+(D−d)/2−x=0.002${V_{3}( \frac{{GAP}\quad{Zone}\quad 3}{{GAP}\quad{Zone}\quad 2} )} = {V_{2} = {1571\quad{in}\text{/}\sec}}$ P₂=300 psiF=(300)(0.0375)=11.25 lbs.F_(right)−F_(left)=4.7 lbs. pushing right

As shown, a force differential of as much as 4.7 lbs, depending on thedegree of validity of the Bernoulli assumption, pushes the inertia mass150 to the right to correct for the off-center condition. As notedabove, preferably the lower portion of the inertia mass 150 alsoincludes a step 205 creating a lower zone 2 and zone 3 (FIG. 12).Accordingly, a centering force acts on the lower portion of the inertiamass 150 when it is off-center from the shaft 134. Therefore, in theexample above, a force of as much as 4.7 lbs also acts on the lowerportion of the inertia mass 150, resulting in a total centering force ofas much as 9.4 lbs acting to center the inertia mass 150 relative to theshaft 134.

For a typical off-road bicycle application, with the inertial masscentered, the ratio of the velocity in zone 2 V₂ to the velocity in zone3 V₃ (i.e., V₂/V₃) is desirably between 0.9 and 0.2. Preferably, theratio of the velocity in zone 2 V₂ to the velocity in zone 3 V₃ isdesirably between 0.8 and 0.35 and more preferably the ratio of thevelocity in zone 2 V₂ to the velocity in zone 3 V₃ is desirably between0.75 and 0.5.

The ratio of the gap G between the shaft 134 and the inertia mass 150 inzone 3 Z₃ and in zone 2 Z₂ (i.e., G₃/G₂), as demonstrated by thecalculations above, influences the magnitude of the self-centering forceproduced by the inertia mass 150. The ratio (G₃/G₂) is desirably lessthan one. If the ratio (G₃/G₂) is equal to one, then by definition thereis no step 205 between zone 2 Z₂ and zone 3 Z₃.

Based on flow continuity from Zone 2 to Zone 3, the ratio of thevelocity V₂ in Zone 2 to the velocity V₃ in Zone 3 (V₂/V₃) is equal tothe ratio of the Gap G₃ at Zone 3 to the Gap G₂ at Zone 2 (G₃/G₂). Inother words, based on flow continuity it follows that:

-   (G₃/G₂)=(V₂/V₃).

Thus, for a typical off-road bicycle application with the inertia masscentered, the ratio of the gap at Zone 3 to the gap at Zone 2 isdesirable between 0.90 and 0.20. Preferably the ratio of the gap at Zone3 to the gap at Zone 2 is desirably between 0.80 and 0.35 and morepreferably the ratio of the gap at Zone 3 to the gap at Zone 2 isdesirably between 0.75 and 0.50.

Advantageously, the self-centering inertia mass 150 is able tocompensate for force differentials due to the manufacturing variationsin the passage 148 size and position as well as transverseaccelerations, all of which tend to push the inertia mass 150off-center. This allows reliable, sensitive operation of the inertiavalve assembly 140 while also permitting cost-effective manufacturingmethods to be employed without compromising performance.

Although a fluid pressure in zone 1 Z₁ of 400 psi was used in the aboveexample, the actual pressure may vary depending on the force exerted onthe shock assembly 38. The upper pressure limit in zone 1 Z₁ istypically determined by the predetermined blow off pressure of the blowoff valve 140. Desirably, for an off-road bicycle rear shock with ashaft diameter of ⅝ inches, the predetermined blow off pressure isapproximately 400 psi. Preferably, the predetermined blow off pressurewithin zone 1 Z₁ is approximately 600 psi and more preferably isapproximately 800 psi. These predetermined blow off pressures areprovided in the context of an off-road bicycle rear shock applicationand may vary for other applications or vehicle types.

FIG. 13 illustrates an alternative arrangement for controlling therefill rate, or timer function, of fluid flow into the pocket 182 as theinertia mass 150 moves in an upward direction away from its closedposition. The end cap 132 includes a channel 208 communicating with anorifice 209 connecting the reservoir chamber 128 and the pocket 182. Theorifice 209 permits fluid to flow between the reservoir chamber 128 andthe pocket 182 in addition to the fluid flow through the clearance C andbleed flow between the check plate 190 and inertia mass 150. The size ofthe orifice 209 may be varied to influence the overall rate of fluidflow into the pocket 182.

FIG. 13 also illustrates an adjustable pocket refill arrangement 210.The adjustable refill arrangement 210 allows external adjustment of therefill rate of fluid flow into the pocket 182. The adjustable refillarrangement includes an inlet channel 212 connecting the reservoirchamber 128 to a valve seat chamber 213. An outlet channel 214 connectsthe valve seat chamber 213 to the pocket 182.

A needle 215 is positioned within the valve seat chamber 213 andincludes a tapered end portion 216, which extends into the outletchannel 214 to restrict the flow of fluid therethrough. External threadsof the needle 215 engage internal threads of the end cap 132 to allowthe needle 215 to move relative to the outlet channel 216. The needle215 includes a seal 217, preferably an O-ring, which creates a fluidtight seal between the needle 215 and the end cap 132. The exposed endof the needle 215 includes a hex-shaped cavity 218 for receiving a hexkey to allow the needle 215 to be rotated. The exposed end of the needle215 may alternatively include other suitable arrangements that permitthe needle 215 to be rotated by a suitable tool, or by hand. Forexample, an adjustment knob may be connected to the needle 215 to allowa user to easily rotate the needle without the use of tools.

Rotation of the needle 215 results in corresponding translation of theneedle 215 with respect to the end cap 132 (due to the threadedconnection therebetween) and adjusts the position of the tapered end 216relative to the outlet channel 214. If the needle 215 is moved inward,the tapered end 216 blocks a larger portion of the outlet channel 214and slows the fluid flow rate into the pocket 182. If the needle 215 ismoved outward, the tapered end 216 reduces its blockage of the outletchannel 214 and speeds the fluid flow rate into the pocket 182. Thispermits user adjustment of the refill rate of the pocket 182 and,accordingly, adjustment of the period of time the inertia mass 150 isheld in an open position. Advantageously, the adjustable refillarrangement 210 allows a user to alter the period of time the inertiavalve 138 is open and thus, the period of lowered compression dampingonce the inertia valve 138 is opened.

FIG. 14 illustrates the suspension fork 34 detached from the bicycle 20of FIG. 1. The suspension fork 34 includes right and left legs 220, 222,as referenced by a person in a riding position on the bicycle 20. Theright leg 220 includes a right upper tube 224 telescoping received in aright lower tube 226. Similarly, the left leg 222 includes a left uppertube 228 telescopingly received in a left lower tube 230. A crown 232connects the right upper tube 224 to the left upper tube 228 therebyconnecting the right leg 220 to the left leg 222 of the suspension fork34. In addition, the crown 232 supports a steerer tube 234, which passesthrough, and is rotatably supported by the frame 22 of the bicycle 20.The steerer tube 234 provides a means for connection of the handlebarassembly 36 to the suspension fork 34, as illustrated in FIG. 1.

Each of the right lower tube 226 and the left lower tube 230 includes adropout 236 for connecting the front wheel 28 to the fork 34. An arch238 connects the right lower tube 226 and the left lower tube 230 toprovide strength and minimize twisting of the tubes 226, 230.Preferably, the right lower tube 226, left lower tube 230, and the arch238 are formed as a unitary piece, however, the tubes 226, 230 and thearch 238 may be separate pieces and connected by a suitable fasteningmethod.

The suspension fork 34 also includes a pair of rim brake bosses 240 towhich a standard rim brake assembly may be mounted. In addition, thefork 34 may include a pair of disc brake bosses (not shown) to which adisc brake may be mounted. Of course, the suspension fork 34 may includeonly one or the other of the rim brake bosses 240 and disc brake bosses,depending on the type of brake systems desired.

FIG. 15 is a cross-section view of the right leg 220 of the suspensionfork 34 having the front portion cutaway to illustrate the internalcomponents of a damping assembly 244 of the fork 34, Preferably, theleft leg 222 of the suspension fork 34 houses any of a known suitablesuspension spring assembly. For example, an air spring or coil springarrangement may be used. In addition, a portion of the suspension springassembly may be housed within the right fork leg 220 along with thedamper assembly 244.

As described previously, the upper tube 224 is capable of telescopicmotion relative to the lower tube 226. The fork leg 220 includes anupper bushing 246 and a lower bushing 248 positioned between the uppertube 224 and the lower tube 226. The bushings 246, 248 inhibit wear ofthe upper tube 224 and the lower tube 226 by preventing direct contactbetween the tubes 224, 226. Preferably, the bushings 246, 248 areaffixed to the lower tube 226 and are made from a self-lubricating andwear-resistant material, as is known in the art. However, the bushings246, 248 may be similarly affixed to the upper tube 224. Preferably, thebushings 246, 248 include grooves (not shown) that allow a small amountof hydraulic fluid to pass between the bushings 246, 248 and the upperfork tube 224 to permit lubrication of the bushing 246 and seal,described below.

The lower tube 226 has a closed lower end and an open upper end. Theupper tube 224 is received into the lower tube 226 through its openupper end. A seal 250 is provided at the location where the upper 224enters the open end of the lower tube 226 and is preferably supported bythe lower tube 226 and in sealing engagement with the upper tube 224 tosubstantially prevent oil from exiting, or a foreign material fromentering the fork leg 220.

The damping assembly 244 is operable to provide a damping force in bothcompression and a rebound direction to slow both compression and reboundmotion of the fork 34. The damper assembly 244 is preferably an openbath, cartridge-type damper assembly having a cartridge tube 252 fixedwith respect to the closed end of the lower tube 226 and extendingvertically upward. A damper shaft 254 extends vertically downward from aclosed upper end of the upper tube 224 and supports a piston 258. Thus,the piston 258 is fixed for movement with the upper tube 224 while thecartridge tube 252 is fixed for movement with the lower tube 226.

The piston 258 is positioned within the cartridge tube 252 and is intelescoping engagement with the inner surface of the cartridge tube 252.A cartridge tube cap 260 closes the upper end of the cartridge tube 252and is sealing engagement with the damper shaft 254. Thus, the cartridgetube 252 defines a substantially sealed internal chamber which containsthe piston 258.

The piston 258 divides the internal chamber of the cartridge tube 252into a variable volume rebound chamber 262 and a variable volumecompression chamber 264. The rebound chamber 262 is positioned above thepiston 258 and the compression chamber 264 is positioned below thepiston 258. A reservoir 266 is defined between the outer surface of thecartridge tube 252 and the inner surfaces of the upper and lower tubes224, 226. A base valve assembly 268 is operably positioned between thecompression chamber 264 and the reservoir 266 and allows selectivecommunication therebetween.

FIG. 16 is an enlarged cross section of the damping assembly 244. Asdescribed above, a cartridge tube cap 260 closes the upper end of thecartridge tube 252. An outer seal 270 creates a seal between thecartridge tube cap 260 and the cartridge tube 252 while an inner seal272 creates a seal between the cartridge tube cap 260 and the dampershaft 254. Accordingly, extension and retraction of the damper shaft 254with respect to the cartridge tube 252 is permitted while maintainingthe rebound chamber 262 in a substantially sealed condition.

The cartridge cap 260 includes a one-way refill valve 274 which, duringinward motion of the damper shaft 254 with respect to the cartridge tube252, allows fluid flow from the reservoir 266 into the rebound chamber262. The refill valve 274 comprises one or more axial passages 276through the cap 260 which are closed at their lower end by refill shimstack 278. Thus, the shim stack 278 allows fluid flow from the reservoir266 to the rebound chamber 262 with a relatively small amount ofresistance. When the fluid pressure in the rebound chamber 262 isgreater than the fluid pressure in the reservoir 266, such as duringretraction of the damper shaft 254, the refill shim stack 278 engagesthe lower surface of the cartridge tube cap 260 to substantially sealthe refill passages 276 and prevent fluid from flowing therethrough.

The piston 258 is fixed to the end of the damper shaft 254 by a threadedfastener 280. The piston includes an outer seal 282 which engages theinner surface of the cartridge tube 252 to provide a sealing engagementbetween the piston 258 and the inner surface of the cartridge tube 252.Thus, fluid flow around the piston is substantially eliminated.

The piston 258 includes a one-way rebound valve assembly 284 whichpermits fluid flow from the rebound chamber 262 to the compressionchamber 264 while preventing flow from the compression chamber 264 tothe rebound chamber 262. The rebound valve assembly 284 comprises one ormore axial passages 286 through the piston 258 closed at their lower endby a rebound shim stack 288. Fluid is able to flow from the reboundchamber 262 through the passages 286 and into the compression chamber264 against the resistance offered by the shim stack 288. When thepressure is greater in the compression chamber 264 than in the reboundchamber 262, the shim stack 288 engages the lower surface of the piston258 to substantially seal the passages 286 and prevent the flow of fluidtherethrough.

In the illustrated embodiment, the cartridge tube 252 is split into anupper portion 290 and a lower portion 292, which are each threadablyengaged with a connector 294 to form the cartridge tube 252. Optionally,a one-piece cartridge tube may be employed. A base member 296 is fixedto the closed end of the lower tube 226 and supports the cartridge 252.The lower portion 292 of the cartridge tube 252 is threadably engagedwith the base member 296.

FIG. 17 is an enlarged cross-sectional view of the base valve assembly268. The base valve assembly 268 is housed within the lower portion 292of the cartridge tube 252 and is supported by a shaft 298 which extendsin an upward direction from the base member 296. The entire base valveassembly 268 is secured onto the shaft 298 by a bolt 300 whichthreadably engages the upper end of the shaft 298.

The base valve assembly 268 includes a compression valve 302, a blowoffvalve 304, and an inertia valve 306. The compression valve 302 ispositioned on the upper portion of the shaft 298. The blowoff valve 304is positioned below the compression valve 302 and spaced therefrom. Thecompression valve 302 and the blowoff valve 304 define a blowoff chamber308 therebetween. A plurality of passages 310 connect the blowoffchamber 308 to a central passage 312 of the base valve shaft 298.

A snap ring 314, which is held in an annular recess of the shaft 298,supports the compression valve 302. A washer 316 positioned underneaththe bolt 300 holds the compression valve 302 onto the shaft 298. Thecompression valve 302 includes a compression piston 318 sealinglyengaged with the inner surface of the lower portion 292 of the cartridgetube 252 by a seal 320. The compression piston 318 is spaced from boththe snap ring 314 and the washer 316 by a pair of spacers 322, 324respectively.

The compression piston 318 includes one or more compression passages 326covered by a compression shim stack 328. The compression shim stack 328is secured to the lower surface of the compression piston 318 by thelower spacer 322. The compression shim stack 328 deflects about thelower spacer 322 to selectively open the compression passages 326. Thecompression shim stack 328 seals against the lower surface of thecompression piston 318 to prevent unrestricted compression flow past thecompression shim stack 328.

As illustrated in FIGS. 20 and 21, which show fluid flows during therebound stroke, the compression piston 318 also includes one or morerefill passages 330 extending axially through the compression piston318. The refill passages 330 are covered at the upper surface of thecompression piston 318 by a refill shim stack 332. The refill shim stack332 is held against the upper surface of the compression piston 318 bythe upper spacer 324 and deflects to open the refill passages 330. Thus,the refill shims 332 prevent fluid flow through the refill passages fromthe compression chamber 264 to the blowoff chamber 308, but permit fluidflow from the blowoff chamber 308 through the refill passages 330 andinto the compression chamber 264 against the slight resistance offeredby the refill shim stack 332.

As illustrated in FIG. 17, the blowoff valve 304 is positioned between alower snap ring 334 and an upper snap ring 336. A separator plate 338 issupported by the lower snap ring 334 and is sealingly engaged with theinner surface of the lower portion 292 of the cartridge tube 252 by aseal 340. A lower spacer 342 spaces the blowoff piston 344 in an upwarddirection from the separator plate 338. The blowoff piston 344 is alsosealingly engaged with the inner surface of the lower portion 292 of thecartridge tube 252 by a seal 346. An upper spacer 348 spaces the blowoffpiston 344 from the upper snap ring 336. A separator chamber 350 isdefined between the blowoff piston 344 and the separator plate 338.

As illustrated in FIGS. 20 and 21, the blowoff piston 344 includes oneor more blowoff passages 352 covered on the lower surface of the blowoffpiston 344 by a blowoff shim stack 354. The blowoff shim stack 354 ispositioned between the blowoff piston 344 and the lower spacer 342 toallow fluid flow from the blowoff chamber 308 into the separator chamber350 at pressures above a predetermined threshold. The blowoff shim stack354 seals passages 352 to prevent unrestricted (without blowoff)compression fluid flow from the blowoff chamber 308 to the separatorchamber 350.

The blowoff piston 344 also includes one or more refill passages 356covered at the upper surface of the blowoff piston 344 by a refill shimstack 358. The refill shim stack 358 is held against the upper surfaceof the blowoff piston 344 by the upper spacer 348 to seal the refillpassages 356 and prevent fluid flow from the blowoff chamber 308 intothe separator chamber 350. However, the refill shims deflect about theupper spacer 348 to allow fluid flow from the separator chamber 350 intothe blowoff chamber 308 through the refill passages 356 with relativelylittle resistance. One or more passages 360 are formed within the lowerportion 292 of the cartridge tube 252 at a height between the separatorplate 338 and the blowoff piston 344 to allow fluid communicationbetween the separator chamber 350 and the reservoir 266.

Preferably, the inertia valve 306 is substantially identical to theinertia valve previously described in relation to the shock absorber 38.The inertia valve 306 includes an inertia mass 362 movable between aclosed position, where the inertia mass 362 closes two or more passages364, and an open position, where the inertia mass 362 uncovers the twoor more passages 364. The uppermost or closed position of the inertiamass 362 is defined by the snap ring 334, which supports the separatorplate 338.

The inertia mass 362 is biased into its closed position by a spring 366.The lowermost or open position of the inertia mass 362 is defined whenthe lower surface of the inertia mass 362 engages the lower interiorsurface of a pocket 368, defamed by the base member 296. The inertiamass 362 includes one or more axial passages 370 covered at the uppersurface of the inertia mass 362 by a check plate 372 which is movablebetween a substantially closed position against the standoff feet 394 atthe upper surface of the inertia mass 362 and an open position againstthe stop projections 392 on the upper, necked portion of the inertiamass 362.

The check plate 372 moves into an open position when the inertia mass362 moves downward in relation to the base valve shaft 298 to allowfluid to flow from the pocket 368 into an inertia valve chamber 376above the inertia mass 362 through the passages 370. The check plate 372moves into a substantially closed position upon upward movement of theinertia mass 362 relative to the base valve shaft 298 to restrict fluidflow through the passages 370. One or more passages 378 are defined bythe lower portion 292 of the cartridge tube 252 to allow fluidcommunication between the inertia valve chamber 376 and the reservoir266.

An annular clearance C is defined between the inertia mass 362 and thepocket 368 when the inertia mass 362 is in its open position. In asimilar manner to the inertia valve described in relation to the shockabsorber 38, the clearance C restricts fluid flow from the inertia valvechamber 376 into the pocket 368. The inertia valve 306 preferablyincludes other features described in relation to the inertia valve ofthe shock absorber 38. For example, the inertia mass 362 preferablyincludes a plurality of standoff feet 394 at the locations discussedabove in relation to the inertia mass of the shock absorber 38.Additionally, the inertia mass 362 includes an annular recess 380aligned with the passages 364 when the inertia mass 362 is in its closedposition. The inertia mass 362 also includes a step preferably on eachend of the interior surface of the inertia mass 362 which is slidingengagement with the base valve shaft 298, as described above. As shown,the inertia mass 362 also includes a labyrinth seal arrangementsubstantially as described above.

When the front wheel 28 of the bicycle 20 of FIG. 1 encounters a bump, aforce is exerted on the fork 34, which tends to compress the fork legs224, 226 in relation to each other. If the upward acceleration of thelower fork tube 226 along its longitudinal axis (i.e., the axis oftravel of the inertia mass 362) is below a predetermined threshold, theinertia mass 362 remains in its closed position. Pressure within thecompression chamber 264 causes fluid to flow through the compressionpassages 326 and into the blowoff chamber 308. If the pressure withinthe blowoff chamber 308 is below a predetermined threshold, the blowoffshims 354 remain closed and the suspension fork 34 remains substantiallyrigid.

If the pressure within the blowoff chamber 308 exceeds the predeterminedthreshold, the blowoff shim stack 354 deflects away from the blowoffpiston 344 to allow fluid to flow through the blowoff passage 352 intothe separator chamber 350 and into the reservoir through the passages360, as illustrated in FIG. 17. Thus, the fork 34 is able to compresswith the compression damping rate being determined primarily by the shimstack 354 of the blowoff piston 344.

As the upper fork leg 224 moves downward with respect to the lower forkleg 226, and thus the piston 258 and damper shaft 254 move downward withrespect to the cartridge 252, fluid is drawn into the rebound chamber262 through the refill valve 274, as illustrated in FIG. 16.

When the upward acceleration of the lower fork leg 226 exceeds apredetermined threshold, the inertia mass 362 tends to stay at rest andovercomes the biasing force of the spring 366 to open the passages 364.Thus, fluid flow is permitted from the central passage 312 of the basevalve shaft 298 into the inertia chamber 376 through the passages 364and from the inertia chamber 376 into the reservoir 266 through thepassages 378, as illustrated in FIGS. 18 and 19. Accordingly, atpressures lower than the predetermined blowoff pressure, when theinertia mass 362 is open (down) fluid is permitted to flow from thecompression chamber 264 to the reservoir 266 and the suspension fork 244is able to compress.

Upon rebound motion of the suspension fork 34, the refill valve 274closes and the fluid within the rebound chamber 262 is forced throughthe rebound passages 286 of the piston 258 against the resistive forceof the rebound shim stack 288, as illustrated in FIG. 20. A volume offluid equal to the displaced volume of the damper shaft 254 is drawninto the compression chamber 264 from the reservoir chamber 266 via thepassages 356 and 330 against the slight resistance offered by the refillshims 358 and 332, as illustrated in FIG. 21.

FIGS. 22-25 illustrate an alternative embodiment of the suspension fork34. The embodiment of FIGS. 22-25 operates in a substantially similarmanner as the suspension fork 34 described in relation to FIGS. 14-21with the exception that the embodiment of FIGS. 22-25 allows flowthrough a compression valve 382 in the piston 258 during compressionmotion. This is known as a shaft-displacement type damper, because avolume of fluid equal to the displaced volume of the shaft 254 isdisplaced to the reservoir 266 during compression motion of the fork 34.For reference, this compares with the previously-described embodimentwhere the displaced fluid volume equals the displaced volume of the fulldiameter of the piston 258. Flow through the piston 258 into the reboundchamber during compression eliminates the need for refill passages inthe cartridge cap, and thus a solid cap 260 is utilized.

The compression valve 382 is a one-way valve, similar in construction tothe one-way valves described above. The compression valve 382 comprisesone or more valve passages 384 formed axially in the piston 258 and ashim stack 386 closing the valve passages 384. As is known, the shimstack 386 may comprise one or more shims. The shims may be combined toprovide a desired spring rate of the shim stack 386. The shim stack 386is deflected to allow fluid flow between the compression chamber 264 andthe rebound chamber 262 during compression of the suspension fork 34.Preferably, shim stack 386 is significantly “softer” than shim stack 328in the base valve assembly 268, in order to ensure sufficient pressurefor upward flow through piston 258 into rebound chamber 262 duringcompression strokes.

The operation of the suspension fork 34 of FIGS. 22-25 is substantiallysimilar to the operation of the suspension fork 34 described in relationto FIGS. 14-21. However, during compression motion of the fork 34 ofFIGS. 22-25, fluid flows from the compression chamber 264 to the reboundchamber 262. This results in less fluid being displaced into thereservoir 266 than in the previous embodiment. As will be appreciated byone of skill in the art, FIGS. 22 and 23 illustrate compression fluidflow when the blow off valve 304 is open. FIGS. 24 and 25 illustratecompression fluid flow when the inertia valve 306 is open.

As will be appreciated by one of ordinary skill, the illustratedsuspension fork and rear shock absorber arrangements advantageouslyminimize unintended movement of the inertia mass 150 due to normalcompression and rebound fluid flow. With particular reference to FIG. 3b, compression fluid flow (illustrated by the arrow in FIG. 3 b) throughthe blow off valve 140 of the rear shock absorber 38 occurs through thepassage 136 of the reservoir shaft 134 as it passes the inertia mass150. Accordingly, fluid moving with any substantial velocity does notdirectly contact the inertia mass 150, thereby avoiding undesiredmovement of the inertia mass 150 due to forces from such a flow.Similarly, compression fluid flow through the passages 148 when theinertia mass 150 is in an open position (FIG. 5) and refill fluid flowupon rebound of the shock absorber 38 are similarly insulated from theinertia mass 150. With reference to FIGS. 17, 19 and 21, the inertiamass 150 is also insulated from contact with moving fluid in thesuspension fork 34. FIGS. 23 and 25 illustrate similar flow paths forthe second embodiment of the suspension fork 34.

FIG. 26 is a graph illustrating the influence of a change in theinternal diameter D of a specific inertia mass 150 on the pressuredifferential between the right and left side when the inertia mass 150is off-center by a distance x of 0.001 inches. As described above inrelation to FIGS. 11 and 12, the reservoir shaft 134, which defines anaxis of motion for the inertia mass 150, has a diameter referred to bythe reference character “d.” The reference character “B” refers to thesize of the step 205, or the difference in the radial dimensions of theinner surface of the inertia mass 150 between zone 2 Z₂ and zone 3 Z₃.For the purposes of illustration in the graph of FIG. 26, the diameter dof the shaft 134 is given a value of 0.375 inches. The step size B isgiven a value of 0.001 inches.

In the graph of FIG. 26, the value of the minimum internal diameter ofthe inertia mass 150 (i.e., the diameter at zone 3 Z₃) is varied and thecorresponding pressure differential between the left and right sides isillustrated by the line 388, given the constants d, B and x. Asdescribed above, the self-centering force is proportional to thepressure differential produced by the design of zones 1, 2 and 3 of theself-centering inertia mass 150. Thus, as the pressure differentialincreases, so does the ability of the inertia mass 150 to center itselfwith respect to the shaft 134. As illustrated, the value of the pressuredifferential between the left and right sides varies greatly withrelatively small changes in the internal diameter D of the inertia mass150. The pressure differential is at its maximum value on the graph whenthe difference between the inertia valve diameter D and the shaftdiameter d is small. The pressure differential diminishes as thedifference between the inertia valve diameter D and the shaft diameter dincreases.

For example, when the inertia valve diameter D is equal to 0.400 inches,the pressure differential is equal to approximately 8 psi. With theinertia valve diameter D equal to 0.400 inches and the shaft diameter dequal to 0.375 inches, the total gap at zone 3 G₃ for both the left andright sides is equal to 0.025 inches (0.400-0.375), when the inertiamass 150 is centered. Accordingly, each gap at zone 3 for the left andright side, G_(3L) and G_(3R), is equal to 0.0125 inches ( 0.025/2),when the inertia mass 150 is centered (FIG. 11).

The pressure differential has substantially increased at a point whenthe inertia valve diameter D is equal to 0.385. At this point, theresulting pressure differential is approximately 38 psi. Following thecalculation above, each gap at zone 3 for the left and right side,G_(3L) and G_(3R), is equal to 0.005 inches, with a centered inertiamass 150.

The pressure differential has again substantially increased, toapproximately 78 psi, at a point when the inertia valve diameter D isequal to 0.381 inches. When the inertia diameter D is equal to 0.381inches, each gap at zone 3 for the left and right side, G_(3L) andG_(3R), is equal to 0.003 inches, assuming the inertia mass 150 iscentered about the shaft 134. At a point when the inertia valve diameterD is equal to 0.379, the pressure differential has increasedsignificantly to approximately 125 psi. At this point, the gap at zone 3for the left and right side, G_(3L) and G_(3R), is 0.002 inches.

The illustrated pressure differential reaches a maximum when the inertiavalve diameter D is equal to 0.377 inches. At this value of D, thepressure differential is approximately 180 psi and each gap at zone 3for the left and right side, G_(3L) and G_(3R), is equal to 0.001inches, again assuming a centered inertia mass 150 and the values of d,B and x as given above. Although the gap at zone 3 G₃ may be reducedfurther, resulting in theoretically greater self-centering forces, a gapin zone 3 G₃ of at least 0.001 inches is preferred to allow the inertiamass 150 to move freely on the shaft 134. A gap G3 below this value mayallow particulate matter within the damping fluid to become trappedbetween the inertia mass 150 and shaft 134, thereby inhibiting orpreventing movement of the inertia mass 150.

FIG. 27 is a graph illustrating the relationship between the size B ofthe “Bernoulli step” 205 and the resulting pressure differentialpercentage. A pressure differential of 0% indicates no pressuredifferential, and thus no self-centering force, is present (i.e., thepressure on the right and left sides of the inertia mass 150 are equal),while a pressure differential of 100% indicates a maximum pressuredifferential, and self-centering force, is present (i.e., zero pressureon one side of the inertia mass 150). The graph is based on a gap atzone 3 G₃ of 0.002 inches, with the inertia mass 150 centered. In otherwords, the inertia mass diameter D minus the shaft diameter d is equalto 0.004 inches, which results in a gap on each of the right and leftsides, G_(3R) and G_(3L), of 0.002 inches.

The graph includes individual lines 390, 392, 394 and 396 representingdifferent off-center values of the inertia valve. The values are givenin terms of the percentage of the total gap G₃ (0.002″ in FIG. 27) thatthe inertia mass 150 is off-center. For example, an off-center amount of25% means that the center axis of the inertia mass 150 is offset 0.0005inches to either the left or right from the center axis of the shaft134. Similarly, an off-center amount of 50% means that the center axisof the inertia mass 150 is offset 0.001 inches from the center axis ofthe shaft 134. Line 390 represents an off-center amount of 25%, line 392represents an off-center amount of 50%, line 394 represents anoff-center amount of 75%, and line 396 represents an off-center amountof 99%.

The largest step size B illustrated on the graph of FIG. 27 is 0.008inches. A step 205 of a larger size B may be provided, however, asindicated by the graphs, theoretical self-centering effects havediminished significantly at this point. Accordingly, the step size isdesirably less than 0.008 inches, at least for off-road bicycleapplications based on these theoretical calculations. The ratio betweenthe gap at zone 3 G₃ and the gap at zone 2 G₂ (i.e., G₃/G₂) in thissituation is 1/5, for a centered inertia mass 150 and a gap at zone 3 G₃of 0.002 inches.

With continued reference to FIG. 27, lines 390-396 illustrate that thepressure differential has increased at a point when the step size B isequal to 0.006 inches in comparison to the pressure differential at astep size B of 0.008 inches. At this point, the ratio between the gap atzone 3 G₃ and the gap at zone 2 G2 (i.e., G₃/G₂), for a centered inertiamass 150, is 1/4. As a result, the self-centering effect is moresubstantial for ratios which are greater than 1/4. The pressuredifferential again increases at a point when the step size B is equal to0.004 inches. At this point, the ratio between the gap at zone 3 G₃ andthe gap at zone 2 G₂ (i.e., G₃/G₂), for a centered inertia mass 150, is1/3. As a result, the self-centering force for ratios aboveself-centering force 1/3 is increased over the self-centering forceobtained with a larger step size B.

For at least a portion of the lines 390-396, the pressure differentialagain increases for step sizes B less than 0.003. At this point, theratio of the gap at zone 3 G₃ to the gap at zone 2 G₂ (i.e., G₃/G₂), fora centered inertia mass 150, is 2/5. Accordingly, the self-centeringeffect is more substantial for ratios which are greater than 2/5.Furthermore, at least a portion of the lines 390-396 illustrate anincrease in the pressure differential at a point when the step size B isequal to 0.002 inches. At this point, the ratio of the gap at zone 3 G₃to the gap at zone 2 G₂ (i.e., G₃/G₂), for a centered inertia mass 150,is 1/2. As a result, the self-centering effect is more substantial forratios which are greater than 1/2.

The graph of FIG. 27 illustrates a general trend that, up to a point,the pressure differential percentage (and self-centering force)increases as the step size B is reduced, especially for large off-centeramounts. However, practical considerations also prevent the size B ofthe step 205 from becoming too small. For example, extremely small stepsizes may be difficult to manufacture, or in the very least, difficultto manufacture for a reasonable cost. Accordingly, the size B of thestep 205 (i.e., G₂-G₃) is desirably greater than, or equal to, 0.0001inches. Preferably, the size B of the step 205 is greater than or equalto 0.001 inches. Additionally, for the practical concerns describedabove, the effectiveness of the self-centering inertia mass 150, atleast theoretically, declines as the step sizes B become too large.Accordingly, the size B of the step 205 is preferably less than 0.002inches. However, as mentioned above, the graph of FIG. 27 is based ontheoretical calculations using Bernoulli's equation, which assumesperfect fluid flow. For actual fluid flows, a much larger step size Bmay be desirable. For example, in actual applications, a step size B of0.02 inches, 0.03 inches, or even up to 0.05 inches is believed toprovide a beneficial self-centering effect. The effectiveness of largerstep sizes B in actual applications is primarily a result of boundarylayers of slow-moving, or non-moving fluid adjacent the inertia mass 150and shaft 134 surfaces resulting in a lower actual flow rate thantheoretically calculated using Bernoulli's equation.

FIG. 28 is a graph, similar to the graph of FIG. 27, illustrating therelationship between the size B of the step 205 and the resultingpressure differential percentage, except that the gap G₃ is 0.001 incheswhen the inertia valve 150 is centered. That is, the inertia massdiameter D minus the shaft diameter d is equal to 0.002 inches, whichresults in a gap on each of the right and left sides, G_(3R) and G_(3L),of 0.001 inches.

The graph includes individual lines representing inertia mass 150off-center values of 25%, 50%, 75% and 99%. Line 400 represents anoff-center amount of 25%, line 402 represents an off-center amount of50%, line 404 represents an off-center amount of 75%, and line 406represents an off-center amount of 99%.

The largest step size B illustrated on the graph of FIG. 28 is 0.008inches. The ratio between the gap at zone 3 G₃ and the gap at zone 2 G₂(i.e., G₃/G₂) in this situation is 1/9, for a centered inertia mass 150and a gap at zone 3 G₃ of 0.001 inches. A step size B of greater than0.008 inches is possible however, as discussed above, at least foroff-road bicycle applications, the step size B is preferably less than0.008 inches based on theoretical calculations.

For at least a portion of the illustrated off-center amounts, thepressure differential increases at a point when the step size B is equalto 0.003 inches. At this point, the ratio between the gap at zone 3 G₃and the gap at zone 2 G₂ (i.e., G₃/G₂), for a centered inertia mass 150,is 1/4. As a result, the centering effect is more substantial for ratioswhich are greater than 1/4. The lines 400-406 illustrate that thepressure differential again increases at a point when the step size B isequal to 0.002 inches. At this point, the ratio between the gap at zone3 G₃ and the gap at zone 2 G₂ (i.e., G₃/G₂), for a centered inertia mass150, is 1/3. As a result, the self-centering effect is greater forratios above 1/3.

The pressure differential again increases for step sizes B less than0.0015. At this point, the ratio of the gap at zone 3 G₃ to the gap atzone 2 G₂ (i.e., G₃/G₂), for a centered inertia mass 150, is 2/5.Accordingly, the centering effect is more substantial for ratios whichare greater than 2/5. Further, the pressure differential increases at apoint when the step size B is equal to 0.001 inches. At this point, theratio of the gap at zone 3 G₃ to the gap at zone 2 G₂ (i.e., G₃/G₂), fora centered inertia mass 150, is 1/2. As a result, the centering effectis more substantial for ratios which are greater than 1/2.

The design parameters of the self-centering inertia mass 150 describedabove, including the size of the gaps G in the different zones (Z₁, Z₂,Z₃) and the size B of the step 205, for example, as well as otherconsiderations, such as the length of time the inertia mass 150 staysopen in response to an activating acceleration force, the spring rate ofthe biasing spring and the mass of the inertia mass 150, for example,may each be varied to achieve a large number of possible combinations.More than one combination may produce suitable overall performance for agiven application. In a common off-road bicycle application, thecombination desirably provides a self-centering force of between 0 and800 lbs. for an off-center amount of 25%. Preferably, a self-centeringforce of between 0 and 40 lbs. is produced and more preferably, aself-centering force of between 0 and 5 lbs. is produced for anoff-center value of 25%. Desirably, the combination provides aself-centering force of at least 0.25 ounces for an off-center amount of25%. Preferably, a self-centering force of at least 0.5 ounces isproduced and more preferably, a self-centering force of at least 1 ounceis produced for an off-center value of 25%. Most preferably aself-centering force of at least 2 ounces is produced for an off-centervalue of 25%. The above values are desirable for a rear shock absorber38 for an off-road bicycle 20. The recited values may vary in otherapplications, such as when adapted for use in the front suspension fork34 or for use in other vehicles or non-vehicular applications.

FIG. 29 illustrates an inertia valve assembly 410, which is similar tothe inertia valve assembly 138 of FIG. 3B. The inertia valve assembly410 of FIG. 29 may be incorporated in a shock absorber, such as theshock absorber 38 of the bicycle 20 illustrated in FIG. 1. The inertiavalve assembly 410 desirably includes an inertia mass 412, which has anincreased density in comparison to the inertia mass 150 of FIG. 3B. As aresult, the inertia mass 412 is more responsive to an acceleration forceof a given magnitude. Preferably, the inertia valve assembly 410operates in a substantially similar manner to the inertia valvearrangement 138 described above and, therefore, the inertia valveassembly 410 and associated shock absorber are described in limiteddetail.

Preferably, the inertia valve assembly 410 is disposed within areservoir tube 414 and is operable to selectively permit fluid flowbetween a first fluid chamber 416 and a second fluid chamber 418. In apreferred embodiment, the first fluid chamber 416 comprises acompression chamber of the shock absorber and the second fluid chamber418 comprises a reservoir chamber of the shock absorber. Preferably, theinertia mass 412 is supported for axial movement on an axis A_(C), whichis defined by a shaft 420. The inertia mass 412 is biased in an upwarddirection (with respect to the orientation of the tube 414 illustratedin FIG. 29) against an upper stop, defined by snap ring 422, by abiasing member, such as coil spring 424. In this position, the inertiamass 412 closes openings 434 in the shaft 420 to define a closedposition of the inertia valve assembly 410.

A base 426 is coupled to a lower end of the reservoir tube 414 and,preferably, includes a cavity 428, which defines a pocket 430 below theinertia mass 412. The pocket 430 is sized and shaped to receive at leasta lower portion of the inertia mass 412. A bottom surface of the cavity432 functions as a lower stop for the inertia mass 412. As described indetail above, preferably, the inertia mass 412 is responsive to anappropriate acceleration force input above a predetermined threshold.Upon being subjected to such an acceleration force, the inertia mass 412moves downwardly relative to the shaft 420, against the biasing force ofthe spring 424, and into the pocket 430. In this position, the inertiamass 412 uncovers openings 434 to permit fluid flow from the first fluidchamber 416 to the second fluid chamber 418 and define an open positionof the inertia valve assembly 410.

The inertia valve assembly 410 also includes a refill valve assembly436, which preferably is configured to at least partially control a flowof fluid between the reservoir chamber 418 and the pocket 430. In theillustrated embodiment, the valve assembly 436 includes a plurality ofhooks 438 (only one shown) extending in an upward direction from thebase 426. Preferably, the hooks 438 are disposed around the periphery ofthe cavity 428 adjacent an inner surface of the reservoir tube 414. In apreferred arrangement, four such hooks 438 are equally spaced around aperiphery of the cavity 428.

The hooks 438 define an upper stop surface 440 and an upper surface ofthe base 426 defines a corresponding lower stop surface 442. A checkplate 444 is retained for movement between the upper stop surface 438and the lower stop surface 442. Preferably, the check plate 444 issubstantially annular in shape with an inner diameter which is slightlylarger than an outer diameter of an adjacent portion of the inertia mass412, such that a clearance distance C is defined therebetween.

In a preferred arrangement, the check plate 444 is configured torestrict a flow of fluid from the reservoir chamber 418 into the pocket430 at a first level and permit fluid flow from the pocket 430 to thereservoir 418 at a second level, which preferably is greater than thefirst level. In operation, when the inertia mass 412 is moving downwardrelative to the shaft 420, such as due to an appropriate accelerationforce, the movement of fluid out of the pocket 430 lifts the check plate444 in an upward direction against the upper stop surface 440, asillustrated in phantom. Accordingly, a large amount of fluid ispermitted to be displaced from the pocket 430 to the reservoir chamber418, as illustrated by the phantom flow line 445.

Conversely, when the inertia mass 412 is moving from a lower mostposition, within the pocket 430, toward the upper stop 422, fluid withinthe reservoir 418 attempts to fill the pocket 430 thereby urging thecheck plate 444 against the lower stop surface 442, as illustrated bythe solid line position of the check plate 444. In the lower position ofthe check plate 444, fluid is restricted to entering the pocket 430 bypassing through the clearance distance C between an inner surface of thecheck plate 444 and an outer surface of the inertia mass 412, asillustrated by the solid flow line 446. Preferably, with such anarrangement, the flow into the pocket 430 is restricted to a rate thatis lower than the rate in which fluid may exit the pocket 430.Accordingly, the inertia mass 412 may move quickly in a downwarddirection into the pocket 430, while movement in an upward direction isslowed to delay the closing of the inertia valve 410 in order to extendthe reduced-damping mode of the shock absorber, as described in detailabove.

Desirably, the inertia mass 412 is configured to have a relatively highdensity, and thus a high mass for a given volume, so that the inertiamass 412 moves more easily through the damping fluid within the chambers418 and 430 to increase the responsiveness of the inertia valve 410 toacceleration force inputs. Preferably, the inertia mass 412 includes afirst section, comprising a first material, and a second section,comprising a second material having a greater density than the firstmaterial. Desirably, the second material has a density greater thanabout 10 g/cm³ and, preferably, greater than about 15 g/cm³. Morepreferably, the second material has a density of about 19 g/cm³. In theillustrated arrangement, the inertia mass 412 comprises a body portion450, which defines an annular cavity 452 filled with a high densitymaterial 454, so as to increase the overall mass of the inertia mass 412without increasing the volume that it occupies. A presently preferredhigh density material 454 is tungsten, preferably in a powdered form,

In addition, the ratio of the mass of the inertia mass 412 to thesurface area of a lowermost surface 456 of the inertia mass 412, normalto the axis A_(C), is also increased in comparison to the previouslydescribed inertia mass constructions. The surface 456 may be defined asa leading surface of the inertia mass 412 when the inertia mass 412 ismoving in a downward direction (i.e., toward the open position).Accordingly, the leading surface area includes a surface 456 a ofstandoff feet 455, which is generally parallel with the surface 456 andperpendicular to the axis A_(C) of the shaft 420. Due to the increasedmass to volume, and mass to leading surface area ratios, the inertiamass 412 more easily displaces fluid from the pocket 430 to move morequickly toward the open position in response to suitable accelerationforce inputs.

In a preferred arrangement, a threaded cap 458 closes an open, upper endof the cavity 452 to retain the tungsten 454 within the cavity. Aperipheral edge of the cap 458 includes external threads 460, which matewith internal threads 462 of the cavity 452. Thus, the cavity 452 may befilled with tungsten 454, or another high density material, and closedwith the threaded cap 458.

The embodiment illustrated in FIG. 29 is preferred at least because themain body portion 450 of the inertia mass 412 may be made from arelatively dense, yet readily processable material, such as brass forexample, while permitting a material with even higher density, such astungsten powder, to be held within the cavity 452 without the need forit to be formed or otherwise processed. Alternatively, the entireinertia mass 412 may be made from a material having higher density thanbrass, such as solid tungsten for example. In a preferred embodiment,the cavity 452, and thus the tungsten powder 454 or other high densitymaterial, occupies a significant portion of the total volume of theinertia mass 412. For example, desirably the high density materialoccupies at least one-third volume of the inertia mass 412. Preferably,the high density material occupies at least one-half and, morepreferably, at least two-thirds of the volume of the inertia mass 412.However, other ratios between the material comprising the main body 450and the material within the cavity 452 may also be used.

An inertia mass configured substantially as described above providesadvantages mass to surface area, or mass to volume, ratios so that theinertia mass is very responsive to acceleration force inputs. The tablesbelow illustrate the change in mass to surface area and mass to volumeratios for a constant volume inertia mass and a constant mass inertiamass, respectively, having varying relative volumes of brass andtungsten. In generating the tables, the annular inertia mass was assumedto have a length of 0.875 inches, an inner diameter of 0.375 inches and,for the constant volume inertia mass, an outer diameter of one (1) inch.For the constant mass inertia mass, the outer diameter (and, thus, theleading surface area) varies. The density of brass was assumed to be8.5539 g/cm³ and the density of tungsten was assumed to be 19.3 g/cm³.The constant volume inertia mass was assumed to have a volume of 9.685cm³ and the constant mass inertia mass was assumed to have a mass of 83grams. The ratios are provided in grams/cubic inch for mass to volumeand grams/square inch for mass to surface area. TABLE 1 Constant Volume% Tungsten 0 10 20 30 40 50 60 70 80 90 100 Mass 83 93 104 114 124 135145 156 166 177 187 Mass/Vol 140 158 175 193 211 228 246 263 281 299 316Mass/Surf. 123 138 154 169 184 200 215 231 246 262 277 Area

TABLE 2 Constant Mass % Tungsten 0 10 20 30 40 50 60 70 80 90 100 Volume9.7 9.2 8.6 8.1 7.5 7.0 6.5 5.9 5.4 4.8 4.3 Mass/Vol. 140 148 158 168180 194 210 230 253 281 316 Mass/Surf. Area 123 130 138 147 158 170 184201 221 246 277

FIGS. 30, 31A and 31B illustrate an alternative inertia mass 470, whichpreferably is configured to provide increased flow resistance, or drag,when moving in a first direction compared to the flow resistance whenmoving in a second, or opposite direction. In a preferred arrangement,the inertia mass 470 includes one or more collapsible drag members 472,which are configured to assume a first orientation when the inertia mass470 is moving in a first direction and a second orientation when theinertia mass 470 is moving in the opposite direction.

As in the inertia valve assemblies described above, the inertia mass 470is supported for axial movement on a shaft 474 within a reservoirchamber 476. In the illustrated embodiment, the inertia mass 470includes a body portion 478, the outer surface of which defines a pairof annular grooves 480. The annular grooves support the drag members472, which are also annular in shape. In a preferred arrangement, thedrag members 472 are constructed from a flexible material, such asrubber or plastic, and extend upwardly and outwardly from the outersurface of the body portion 478 of the inertia mass. In addition, thedrag members 472 may curve in an upward direction from an inner diameterto an outer diameter of the drag member 472. Accordingly, a peripheraledge portion of each drag member 472 tends to be collapsible in anupward direction relative to the inner edge portion of the drag member472.

In operation, when the inertia mass 470 is moving in a downwarddirection relative to the shaft 474, or toward an open position, fluidflow illustrated by the arrows 482 in FIG. 31A exerts an upward force onthe drag members tending to collapse the drag members radially inward.Accordingly, a leading surface area of the inertia mass 470 is reducedand the fluid 482 flows past the drag members 472 with, preferably,little interruption. Thus, preferably, the drag members 472 exert littleresistive force against the downward movement of the inertia mass 470toward the open position.

Conversely, when the inertia mass 470 is moving in an upward directionrelative to the shaft 474, toward the closed position, fluid 484 flowingbeside the inertia mass 470 tends to open the drag members 472 intotheir relaxed, or radially extended, orientation, as illustrated in FIG.31B. Thus, preferably, the drag members 472 cause turbulent flow of thefluid adjacent the body portion 478. Such flow significantly increasesthe resistance to fluid 484 flowing past the inertia mass 470 and,thereby, slows the movement of the inertia mass 470 toward the closedposition. Thus, the drag members 472 provide a delay, or timer function,to the inertia mass 470, in a manner similar to the timer arrangementsdescribed above.

The drag members 472 may be used in addition, or in the alternative, toother delay producing devices, such as the valve 436 of FIG. 29 or theclearance passage C illustrated in FIG. 6. Furthermore, although twodrag members 472 are provided in the illustrated inertia valve assembly470, a greater or lesser number of drag members 472 may also be used. Inaddition, although the drag members 472 are illustrated as annularmembers extending outwardly from a side wall of the inertia mass 470,other constructions are also possible. For example, collapsible dragmembers may be disposed above or below the main body 478 of the inertiamass 470 and be configured in a similar manner to achieve the same, orsimilar, effect.

FIG. 32 illustrates an alternative inertia valve assembly 490 in whichthe delay in closing of the inertia mass 492 is influenced by a pressuredifferential between the pressure of the fluid within the reservoirchamber 494 and the pressure of the fluid within the passage 526. Duringa rebound stroke of the shock absorber, as fluid exits the reservoirchamber 494, flowing downward (relative to the orientation shown in FIG.32) through the central shaft 496, a pressure drop occurs. For a givenflow rate, the magnitude of the pressure drop is influenced by thediameter of the flow passage in the shaft 496. A smaller flow passagediameter creates a larger pressure drop top to bottom.

Similar to the previous embodiments, the inertia mass 492 is supportedby a shaft 496 for axial movement about an axis A_(C). The inertia mass492 is positioned within the reservoir chamber 494 defined by areservoir tube 498. A base 500 is connected to a lower end of thereservoir tube 498 and defines a recess 502 which, in turn, defines apocket 504 for receiving at least a lower portion of the inertia mass492 when the inertia mass 492 is in the open position. Thus, a bottomsurface of the recess 502 functions as a lower stop for the inertia mass492. The inertia mass 492 is biased against an upper stop, defined bysnap ring 506, by a biasing member, such as coil spring 508.

Preferably, the base 500 defines a first passage 510 that connects thereservoir chamber 494 and the pocket 504. Desirably, the base 500 alsodefines a second passage 512 that connects the reservoir chamber 494 andthe pocket 504. A pressure actuated valve arrangement 514 selectivelypermits fluid communication through the second passage 512 when thepressure in the reservoir chamber is above a predetermined threshold.The valve assembly 514 includes a valve body 516 biased into a closedposition by a biasing member, such as coil spring 518. In the closedposition, an enlarged diameter upper portion 517 of the valve body isarranged to block the second passage 512 to substantially prevent fluidflow therethrough.

Preferably, an upper stop for the valve body 516 is defined by a snapring 520 and a lower stop is defined by a lower end of a valve seat 521,which receives the upper portion 517 of the valve body 516. Desirably,the valve body 516 includes an elongated lower end, or shaft portion522, which functions as a guide for the coil spring 518. In addition,preferably a seal member 528 creates a seal between the valve body 516and the base 500 to inhibit fluid from passing therebetween. Thus, thevalve body 516 is normally biased into a closed position by the force ofthe biasing member 518. If the pressure differential between thereservoir chamber 494 and the passage 526 exceeds a predeterminedthreshold, the valve body 516 moves toward the open position, againstthe biasing force of the spring 518. In the illustrated arrangement, thepredetermined threshold is determined primarily by the surface area ofthe upper end surface of the valve body 516 and the spring constant ofthe biasing member 518,

As described above, when the inertia mass 492 moves into its openposition, refilling of the pocket 504 is restricted to fluid flowbetween an outer surface of the inertia mass 492 and an inner surface ofthe cavity 502. In addition, fluid may refill the pocket 504 by flowingthrough the passage 510, if provided. Thus, the inertia mass 492 isdelayed from moving toward its open position due to the restriction ofthe fluid from entering the pocket 504. However, in the embodiment ofFIG. 32, if the pressure differential between the reservoir chamber 494and the passage 526 exceeds a predetermined threshold, the pressureactuated valve assembly 514 opens to permit fluid flow into the pocket504 through the second passage 512. Preferably, the second passage 512is configured to permit a greater rate of flow into the pocket 504 incomparison to fluid flow through the clearance between the inertia mass492 and the cavity 502 and fluid flow through the passage 510 (ifprovided). Accordingly, when the pressure actuated valve assembly 514opens, the inertia mass 492 may return to its closed position morequickly.

FIG. 33 illustrates an alternative embodiment of a pressure activatedinertia valve assembly 530. In the embodiment of FIG. 33, an inertiamass 532 is configured for axial movement on a shaft 534 about an axisA_(C). Preferably, the inertia mass 532 is disposed within a reservoirchamber 536 defined at least partially by a reservoir tube 538 and abase 540. A passage 542 extends through the base 540 and shaft 534 andis in fluid communication with the reservoir chamber 536 throughopenings 544. Desirably, the passage 542 receives fluid from acompression chamber (not shown) of the shock absorber, as will beappreciated by one of skill in the art, Thus, the inertia mass 532selectively permits fluid communication between the passage 542 and thereservoir chamber 536.

In the embodiment of FIG. 33, a slide member 546 is interposed betweenthe base 540 and the inertia mass 532. The slide 546 includes a recess548 that defines a pocket 550 for receiving the inertia mass 532. Theinertia mass 532 is biased into an uppermost, or closed, position(against stop 552) by a biasing member, such as coil spring 554. Thespring 554 is supported relative to the shaft 534 by a lower stop,defined by snap ring 556. The snap ring 556 also defines an uppermostposition of the slide 546. The slide 546 is also axially moveablyrelative to the shaft 534 and is biased into its uppermost position by abiasing member, such as coil spring 558.

The base 540 defines a cavity 560, which receives a lower end of theslide 546 in a sealed arrangement. One of a lower surface 562 of thecavity 560 or an upper surface 564 of the base 540 function as a stop todefine a lowermost position of the slide 546. In addition, preferablyone or more passages 566 permit fluid communication between the passage542 and a pocket 568 defined by the cavity 560. Preferably, the pocket568 is substantially sealed, with the exception of the passages 566,such that fluid within the pocket 568 is at substantially the samepressure as fluid within the passage 542 (and, thus, the compressionchamber of the shock absorber).

In operation, the inertia mass 532, upon receiving an appropriateacceleration force, moves in a downward direction relative to the shaft534 and into the pocket 550. Once in the pocket 550, the inertia mass532 is delayed in moving in an upward direction due to the restrictionof fluid being permitted to refill the pocket 550. Thus, the inertiamass 532, when positioned within the pocket 550, moves toward the closedposition at a delayed rate. In the illustrated embodiment, fluid maypass from the reservoir chamber 536 into the pocket 550 through aclearance distance C between an outer diameter of the inertia mass 532and an inner diameter of the cavity 548.

When a difference in fluid pressure between the reservoir chamber 536and the passage 542 (and, thus, the pressure within the compressionchamber of the shock absorber) exceeds a predetermined threshold, theslide 546 moves downward relative to the shaft 534 and into the pocket568. In the illustrated embodiment, preferably, the predeterminedthreshold is determined primarily by a surface area of an end surface569 the slide 546, which is perpendicular to the center axis Ac of theshaft 534 and disposed within the pocket 568, along with the spring rateof the biasing member 558,

Thus, with the inertia mass 532 in its open position, the slide 546moves in a downward direction away from the inertia mass 532. When theslide 546 moves downwardly a sufficient distance, the inertia mass 532is no longer present within the pocket 550 and fluid may refill thepocket 550 at a relatively high rate. Thus, the inertia mass 532 is nolonger restricted from moving in an upward direction due to therestriction of fluid moving into the pocket 550 and, as a result, thebiasing member 554 returns the inertia mass 532 to its closed positionat a normal rate, determined primarily by the weight of the inertia mass532 and the spring rate of the spring 554. Accordingly, with such anarrangement, when the inertia mass 532 is in the open position and thepressure within the reservoir chamber 536 exceeds the pressure withinthe passage 542 by a predetermined threshold, the inertia mass 532 ispermitted to return to the closed position without significant delay.

FIGS. 34 and 35 illustrate a bicycle that employs yet anotheralternative embodiment of an acceleration sensitive shock absorber. Thebicycle 580 includes a main frame portion 582, an articulating frameportion 584, a front wheel 586, and a rear wheel 588. Preferably, afront suspension assembly 590 is operably positioned between the frontwheel 586 and the main frame 582 and a rear suspension assembly, orshock absorber 592, is operably positioned between the rear wheel 588and the main frame 582. Preferably, the articulating frame portion 584carries the rear wheel 588 and the shock absorber 592 is connected tothe articulating frame portion 584 to resist movement of the rear wheel588 in an upward direction. Preferably, the shock absorber 592 ispositioned on one lateral side of the rear wheel 588 and, desirably, onthe left-hand side of the rear wheel 588.

With reference to FIG. 35, desirably, the shock absorber 592 includes areservoir chamber 594 at least partially defined by a reservoir tube 596and a base 598. Preferably, an acceleration sensitive valve assembly 600is disposed within the reservoir chamber 594. The valve assembly 600preferably includes a valve body 602 biased into an uppermost, or openposition, by a biasing member, such as coil spring 604. The valve body602 is supported for axial movement along an axis A_(C), which isdefined by a shaft 606. An uppermost position of the valve body 602preferably is determined by a snap ring 608. In the illustratedembodiment, the uppermost position defines a closed position of thevalve 600.

The base 598 preferably includes a cavity 610 that defines a pocket 612in which the valve body 602 enters in its lowermost position. In apreferred arrangement, when the valve body 602 is in its lowermostposition, fluid flow is permitted through openings 613 of the shaft 606.A bottom surface 614 of the cavity 610 defines a lower stop for thevalve body 602. Preferably, as described above, a valve assembly 616 isprovided to permit relatively free flow of fluid from the pocket 612 tothe reservoir chamber 594 while permitting restricted flow of fluid fromthe reservoir chamber 594 into the pocket 612.

Desirably, the valve assembly 600 includes a system for sensingacceleration force inputs and for moving the valve body 602 to an openposition and/or retaining the valve body 602 in an open position. In theillustrated embodiment, preferably an electromagnetic system 618 isprovided. The system 618 preferably includes an electromagnetic forcegenerator 620 within the base 598 and positioned below the valve body602. A control assembly 622 is operably connected to the electromagneticforce generator 620. Preferably, the valve body 602 includes a lowerportion 624, which is constructed from a magnetic material. Theelectromagnetic force generator 620 desirably is configured toselectively apply an attractive force to the magnetic portion 624 of thevalve body 602. Thus, the valve body 602 may be moved toward, orretained in, an open position by the electromagnetic force generator620.

With reference to FIG. 34, preferably, a sensor 626 is positioned on thefront suspension assembly 590 for movement with a hub axis A_(H) of thefront wheel 586. In addition, or in the alternative, a sensor 628 may besecured to the articulating frame portion 584 for movement with a hubaxis A_(H) of the rear wheel 588. Preferably, each of the sensors 626and 628 are configured to sense substantially vertical accelerationforce inputs to the front or rear wheels 586, 588, respectively.

The sensors 626, 628 are configured to communicate with the controlassembly 622 to provide a control signal indicative of the accelerationforces acting on the front or rear wheels 586, 588. In a preferredembodiment, the sensors 626, 628 produce an electronic signal tocommunicate with the control assembly 622. In such an embodiment, thesensors 626, 628 may communication with the control assembly 622 througha hardwired system or, preferably, over a wireless communication system.Furthermore, other suitable types of sensors and methods ofcommunication between the sensors 626, 628 and the control assembly 622may also be used, such as hydraulic or mechanical systems, for example.Thus, the control signal may include changes in hydraulic pressure, ormovement of a mechanical linkage, for example. Other suitable systemsapparent to one of skill in the art may also be used.

The control assembly 622 preferably includes a processor and a memoryfor storing a control algorithm, or protocol. The control assembly 622uses the control signal provided by the sensors 626, 628 along with thecontrol algorithm to determine whether to activate the electromagneticforce generator 620. Thus, when an appropriate acceleration force inputis detected, the control assembly 622 may activate the electromagneticforce generator 620 to move the valve body 602 from its closed positioninto an open position and, if desirable, retain the valve body 602 in anopen position for a period of time, or a delay period.

Desirably, the control assembly 622 includes an adjustment mechanism, topermit adjustment of the delay period in which the valve body 602 isheld in an open position and/or the acceleration force threshold abovewhich the valve assembly 600 is opened. Preferably, the control assembly622 includes a first adjustment knob 630, to permit adjustment of thedelay period, and a second adjustment knob 632, to permit adjustment ofthe acceleration force threshold.

The valve body 602 may be fully controlled by the electromagnetic forcegenerator 620 or may be configured to be self-responsive to accelerationforce inputs due to the inertia of the valve body 602. Furthermore, thevalve 616 may be provided to determine a delay period of the valve body602 or the electromagnetic force generator 620 may be relied on toprovide the delay in the valve body 602 from returning to the closedposition. In addition, a combination of inertia forces andelectromagnetic forces may be utilized to open the valve body 602 and acombination of fluid restriction, or fluid suction, forces andelectromagnetic forces may be utilized to provide the valve body 602with a delay period in moving from an open position to a closedposition.

Advantageously, by positioning the sensor 626 to sense accelerationforce inputs of the front wheel 586, the valve body 602 in the rearshock absorber 592 may be moved into its open position before the object(e.g., such as a bump, rock or other irregularity in the trail surface)which caused the acceleration force is encountered by the rear wheel588. Thus, there is no delay in the altered rate of damping of the rearshock absorber 592 due to the valve body 602 having to move from itsclosed position to its open position upon encountering the bump, orother obstacle, because the bump has been “anticipated” by the sensor626 positioned to detect acceleration of the front wheel 586.

As described above, preferably, the valve body 602 remains in an openposition, or is delayed from returning to its closed position, so thatthe rear wheel 588 may absorb a series of bumps and the valve assembly600 does not have to reactivate upon encountering each individual bump.Advantageously, by permitting the delay to be controlled by theadjustment mechanism 630, a rider can tune the shock absorber 592 tosuit anticipated trail conditions by providing a relatively short or arelatively long delay time. In addition, the acceleration threshold mayalso be adjusted such the size of bump necessary to open the valveassembly may be varied.

Furthermore, the front suspension assembly 590 may also be configured toinclude an acceleration sensitive valve assembly, similar to the valveassembly 600. In addition, the various features illustrated in FIGS.1-35 may be used in combination with one another to provide a desiredresult, as may be determined by one of skill in the art.

Although the present invention has been explained in the context ofseveral preferred embodiments, minor modifications and rearrangements ofthe illustrated embodiments may be made without departing from the scopeof the invention. For example, but without limitation, although thepreferred embodiments described an inertia valve damper for altering therate of compression damping, the principles taught may also be utilizedin damper embodiments for altering rebound damping, or for responding tolateral acceleration forces, rather than vertical acceleration forces.In addition, although the preferred embodiments were described in thecontext of an off-road bicycle application, the present damper may bemodified for use in a variety of vehicles, or in non-vehicularapplications where dampers may be utilized. Furthermore, theself-centering and timer features of the inertia valve assembly may beapplied to other types of valves, which may be actuated by accelerationforces or by means other than acceleration forces. Accordingly, thescope of the present invention is to be defined only by the appendedclaims.

1. A fluid damper, comprising: a hollow shaft, separate from the pistonshaft, defining portions of at least a compression fluid flow path and arebound fluid flow path; an inertia mass movable along the outside ofthe shaft and having a first position for preventing fluid flow througha portion of the compression fluid flow path and a second position forallowing fluid flow through a portion of the compression fluid flowpath; and wherein movement of the inertia mass along the outside of thehollow shaft is substantially isolated from all compression and reboundfluid flows.
 2. The fluid damper of claim 1, wherein: the inertia massincludes an outer surface and the outer surface of the inertia mass issubstantially isolated from all compression and rebound fluid flows. 3.The fluid damper of claim 1, wherein during compression and when theinertia mass is in its second position, the compression fluid flow is ina direction above and away from the inertia mass.
 4. The fluid damper ofclaim 1, wherein the hollow shaft also defines a blow-off fluid flowpath.
 5. A fluid damper, comprising: a damper body; damping fluidcontained within the damper body and that flows in one direction duringa compression fluid flow and flows in a second direction during therebound fluid flow; a valve, including a moveable inertia mass containedwithin the damping fluid, for at least partially controlling thecompression fluid flow and reacting differently to ground-induced andrider-induced forces; and the operation of the valve being substantiallyisolated from all compression and rebound fluid flows within the damperbody.
 6. The fluid damper of claim 5, wherein during operation of thedamper, at least a portion of the compression fluid flow is in adirection above and away from the inertia mass.
 7. A fluid damper,comprising: a reservoir chamber; at least one fluid flow path into thereservoir chamber; at least one fluid flow path out of the reservoirchamber; a moveable inertia mass for controlling fluid flow through theat least one fluid flow path into the reservoir chamber; and movement ofthe inertia mass is substantially isolated from fluid flowing throughboth fluid flow paths.
 8. The fluid damper of claim 7, wherein a hollowshaft defines a portion of the fluid path into the reservoir chamber anda portion of the fluid path out of the reservoir chamber.
 9. The fluiddamper of claim 7, wherein the moveable inertia mass is contained withinthe damping fluid.